Fix: RedBoxes don't show up after teardowns (facebook#38997)

Summary:
Pull Request resolved: facebook#38997

After React Native gets destroyed (e.g: via an exception), the ReactHost resets its current activity.

## Problem
React Native can display RedBoxes after React Native destruction (e.g: in the case of an exception).

Displaying RedBoxes requires the current activity, which gets nullified. So, the RedBox might not show up after destruction.

## Changes
This diff makes ReactHost keep a track of its last non-null activity in a WeakRef.
Then, the DevMenu just uses the last non-null activity to display RedBoxes (and everything else).

Changelog: [Internal]

Differential Revision: https://internalfb.com/D48076893

fbshipit-source-id: 4f06527d7570b212e2bca985dc04912741fa9830

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/bridgeless/BridgelessDevSupportManager.java

@@ -121,7 +121,7 @@ public void toggleElementInspector() {
       @androidx.annotation.Nullable
       @Override
       public Activity getCurrentActivity() {
-        return reactHost.getCurrentActivity();
+        return reactHost.getLastUsedActivity();
       }
 
       @Override

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/bridgeless/ReactHostImpl.java

@@ -117,6 +117,8 @@ public class ReactHostImpl implements ReactHost {
       new BridgelessAtomicRef<>();
 
   private final AtomicReference<Activity> mActivity = new AtomicReference<>();
+  private final AtomicReference<WeakReference<Activity>> mLastUsedActivity =
+      new AtomicReference<>(new WeakReference<>(null));
   private final BridgelessReactStateTracker mBridgelessReactStateTracker =
       new BridgelessReactStateTracker(DEV);
   private final ReactLifecycleStateManager mReactLifecycleStateManager =
@@ -504,8 +506,20 @@ private MemoryPressureListener createMemoryPressureListener(ReactInstance reactI
     return mActivity.get();
   }
 
+  @Nullable
+  /* package */ Activity getLastUsedActivity() {
+    @Nullable WeakReference<Activity> lastUsedActivityWeakRef = mLastUsedActivity.get();
+    if (lastUsedActivityWeakRef != null) {
+      return lastUsedActivityWeakRef.get();
+    }
+    return null;
+  }
+
   private void setCurrentActivity(@Nullable Activity activity) {
     mActivity.set(activity);
+    if (activity != null) {
+      mLastUsedActivity.set(new WeakReference<>(activity));
+    }
   }
 
   /**