chore(Android): remove unused NativeDismissalObserver interface (#2768)

## Description

After recent refactor in Android sheet animation & fragment structure we
no longer use the
`NativeDismissalObserver`.

## Changes

Removed the interface & unused field from `ScreenStack`.

## Test code and steps to reproduce

Code should build.

## Checklist

- [ ] Ensured that CI passes

Author: kkafar

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

@@ -65,7 +65,6 @@ class KeyboardVisible(
 class ScreenStackFragment :
     ScreenFragment,
     ScreenStackFragmentWrapper {
-    public var nativeDismissalObserver: NativeDismissalObserver? = null
     private var appBarLayout: AppBarLayout? = null
     private var toolbar: Toolbar? = null
     private var isToolbarShadowHidden = false