feat(Android): add basic support for nesting v4 stack in new bottom-tabs (#158)

## Description

Closes #159

This PR adds support for nesting v4 stack in new bottom tabs component
on Android.

Currently, an attempt to do so would crash the application with known
error "FragmentManager is already executing transactions".
This is, because both `TabsHost` & `ScreenStack` would use the same
`FragmentManager` (application `supportFragmentManager`).

This PR adds support for **nesting ScreenStack inside TabsHost** (note
that **only for this configuration**),
by adjusting fragment manager finding code in v4 stack implementation
and adding necessary logic in `TabScreen`.

## Changes

Please see particular commits for precise description.

* [Add support for FragmentProviding in v4
code](software-mansion/react-native-screens-labs@d0ca74f)
This allows v4 stack to discover that it is nested inside gamma
container.
The interop is not full, because `ScreenContainer` expects
the parent that provides fragment manager to also provide
`FragmentWrapper` and this is not supported for gamma containers.
This incompatibility prevents communication between v4 & gamma
containers.
I do not recall exactly what won't work, but something surely won't.

* [Add method to TabScreenDelegate providing fragment acccess for
`TabScreen`](software-mansion/react-native-screens-labs@319fe7e)
I don't really like this method being added to the main delegate,
however I want to keep `TabScreen` unaware of its hosting fragment
(as long as possible). Simultaneously, when looking for fragment
manager from nested stack, the `TabScreen` is the place we need to
stop the search at, otherwise `TabsHost` wouldn't know under which tab
the nested stack is hosted. We could do some reasonable assumptions,
e.g. focused tab, however I'm not sure this would work (separate
discussion). Therefore `TabScreen` needs to be aware of the
`fragment` - and we end up with contradiction. This led me to putting
a provider method on the `TabScreenDelegate`. Otherwise I would have to
create another delegate interface & set / remove another delegate.
I guess it would be cleaner, but seemed like an overkill for now.
We might do that later, however.



## Test code and steps to reproduce

I don't want to ruin the current example. We need to extract the
`BottomTabsContainer` & `StackContainer` components, so that they can be
reused across different tests.


## Checklist

- [ ] Ensured that CI passes

Author: kkafar

android/src/main/java/com/swmansion/rnscreens/ScreenContainer.kt

@@ -18,6 +18,7 @@ import com.facebook.react.uimanager.ThemedReactContext
 import com.facebook.react.uimanager.UIManagerHelper
 import com.swmansion.rnscreens.Screen.ActivityState
 import com.swmansion.rnscreens.events.ScreenDismissedEvent
+import com.swmansion.rnscreens.gamma.common.FragmentProviding
 
 open class ScreenContainer(
     context: Context?,
@@ -175,7 +176,7 @@ open class ScreenContainer(
     private fun setupFragmentManager() {
         var parent: ViewParent = this
         // We traverse view hierarchy up until we find screen parent or a root view
-        while (!(parent is ReactRootView || parent is Screen) &&
+        while (!(parent is ReactRootView || parent is Screen || parent is FragmentProviding) &&
             parent.parent != null
         ) {
             parent = parent.parent
@@ -190,6 +191,13 @@ open class ScreenContainer(
                     setFragmentManager(fragmentWrapper.fragment.childFragmentManager)
                 },
             ) { "Parent Screen does not have its Fragment attached" }
+        } else if (parent is FragmentProviding) {
+            // TODO: We're missing parent-child relationship here between old container & new one
+            val fragmentManager =
+                checkNotNull(
+                    parent.getFragment(),
+                ) { "[RNScreens] Parent $parent returned nullish fragment" }.childFragmentManager
+            setFragmentManager(fragmentManager)
         } else {
             // we expect top level view to be of type ReactRootView, this isn't really necessary but in
             // order to find root view we test if parent is null. This could potentially happen also when

android/src/main/java/com/swmansion/rnscreens/gamma/tabs/TabScreen.kt

@@ -2,7 +2,9 @@ package com.swmansion.rnscreens.gamma.tabs
 
 import android.util.Log
 import android.view.ViewGroup
+import androidx.fragment.app.Fragment
 import com.facebook.react.uimanager.ThemedReactContext
+import com.swmansion.rnscreens.gamma.common.FragmentProviding
 import java.lang.ref.WeakReference
 import kotlin.properties.Delegates
 
@@ -11,7 +13,8 @@ import kotlin.properties.Delegates
  */
 class TabScreen(
     val reactContext: ThemedReactContext,
-) : ViewGroup(reactContext) {
+) : ViewGroup(reactContext),
+    FragmentProviding {
     override fun onLayout(
         changed: Boolean,
         l: Int,
@@ -57,6 +60,8 @@ class TabScreen(
         tabScreenDelegate = WeakReference(delegate)
     }
 
+    override fun getFragment(): Fragment? = tabScreenDelegate.get()?.getFragmentForTabScreen(this)
+
     private fun onTabFocusChangedFromJS() {
         tabScreenDelegate.get()?.onTabFocusChangedFromJS(this, isFocusedTab)
     }

android/src/main/java/com/swmansion/rnscreens/gamma/tabs/TabScreenDelegate.kt

@@ -1,10 +1,17 @@
 package com.swmansion.rnscreens.gamma.tabs
 
+import androidx.fragment.app.Fragment
+
 internal interface TabScreenDelegate {
     fun onTabFocusChangedFromJS(
         tabScreen: TabScreen,
         isFocused: Boolean,
     )
 
     fun onMenuItemAttributesChange(tabScreen: TabScreen)
+
+    /**
+     * This returns fragment **if one is associated with given tab screen**.
+     */
+    fun getFragmentForTabScreen(tabScreen: TabScreen): Fragment?
 }

android/src/main/java/com/swmansion/rnscreens/gamma/tabs/TabsHost.kt

@@ -216,6 +216,8 @@ class TabsHost(
         }
     }
 
+    override fun getFragmentForTabScreen(tabScreen: TabScreen): TabScreenFragment? = tabScreenFragments.find { it.tabScreen === tabScreen }
+
     private fun updateBottomNavigationViewAppearance() {
         Log.w(TAG, "updateBottomNavigationViewAppearance")
         bottomNavigationView.isVisible = true