[refactor] Add element type for Activity (#32499)

This PR separates Activity to it's own element type separate from
Offscreen. The goal is to allow us to add Activity element boundary
semantics during hydration similar to Suspense semantics, without
impacting the Offscreen behavior in suspended children.

DiffTrain build for [1a19170](1a19170)

Author: rickhanlonii

compiled-rn/VERSION_NATIVE_FB

@@ -1 +1 @@
-19.1.0-native-fb-c4a3b92e-20250314
+19.1.0-native-fb-1a191701-20250317

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/react-dom/cjs/ReactDOM-dev.js

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<c3d6f2b6c71489013592048f79a0b1b0>>
+ * @generated SignedSource<<f6f36a0c9bac4b50ed9c6c711bbd99c6>>
  */
 
 "use strict";
@@ -404,5 +404,5 @@ __DEV__ &&
     exports.useFormStatus = function () {
       return resolveDispatcher().useHostTransitionStatus();
     };
-    exports.version = "19.1.0-native-fb-c4a3b92e-20250314";
+    exports.version = "19.1.0-native-fb-1a191701-20250317";
   })();

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/react-dom/cjs/ReactDOM-prod.js

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<a90df4bb663907a4d8bc8da3b3606c1f>>
+ * @generated SignedSource<<f53f9e61625be431682d148f397a743b>>
  */
 
 "use strict";
@@ -203,4 +203,4 @@ exports.useFormState = function (action, initialState, permalink) {
 exports.useFormStatus = function () {
   return ReactSharedInternals.H.useHostTransitionStatus();
 };
-exports.version = "19.1.0-native-fb-c4a3b92e-20250314";
+exports.version = "19.1.0-native-fb-1a191701-20250317";

compiled-rn/facebook-fbsource/xplat/js/RKJSModules/vendor/react/react-dom/cjs/ReactDOM-profiling.js

@@ -7,7 +7,7 @@
  * @noflow
  * @nolint
  * @preventMunge
- * @generated SignedSource<<a90df4bb663907a4d8bc8da3b3606c1f>>
+ * @generated SignedSource<<f53f9e61625be431682d148f397a743b>>
  */
 
 "use strict";
@@ -203,4 +203,4 @@ exports.useFormState = function (action, initialState, permalink) {
 exports.useFormStatus = function () {
   return ReactSharedInternals.H.useHostTransitionStatus();
 };
-exports.version = "19.1.0-native-fb-c4a3b92e-20250314";
+exports.version = "19.1.0-native-fb-1a191701-20250317";