Fixes a race condition when merge animation transition

When merge a TO_FRONT transition to the recents animation,
onTasksAppeared will finish the recents animation, so there can be a
race that the open transition for recents animation might be finish
before the merge transition, which leave the merge transition become
a standalone transition.
To make the sequence correct, register a transition commit callback
to know the merge transition has done in Shell, then goes to finish
the recents animation.

Bug: 235433368
Test: enable shell transitin, quick switch back/forward and verify no
flicker.

Change-Id: I39fc898a8922f900e1e1808f92a471749664e194

packages/SystemUI/shared/src/com/android/systemui/shared/system/RemoteTransitionCompat.java

@@ -186,6 +186,8 @@ public class RemoteTransitionCompat implements Parcelable {
                 } catch (RemoteException e) {
                     Log.e(TAG, "Error merging transition.", e);
                 }
+                // commit taskAppeared after merge transition finished.
+                mRecentsSession.commitTasksAppearedIfNeeded(recents);
             }
         };
         mTransition = new RemoteTransition(remote, appThread);
@@ -226,6 +228,7 @@ public class RemoteTransitionCompat implements Parcelable {
         private PictureInPictureSurfaceTransaction mPipTransaction = null;
         private IBinder mTransition = null;
         private boolean mKeyguardLocked = false;
+        private RemoteAnimationTargetCompat[] mAppearedTargets;
 
         void setup(RecentsAnimationControllerCompat wrapped, TransitionInfo info,
                 IRemoteTransitionFinishedCallback finishCB,
@@ -251,6 +254,7 @@ public class RemoteTransitionCompat implements Parcelable {
         boolean merge(TransitionInfo info, SurfaceControl.Transaction t,
                 RecentsAnimationListener recents) {
             SparseArray<TransitionInfo.Change> openingTasks = null;
+            mAppearedTargets = null;
             boolean cancelRecents = false;
             boolean homeGoingAway = false;
             boolean hasChangingApp = false;
@@ -331,10 +335,17 @@ public class RemoteTransitionCompat implements Parcelable {
                 targets[i] = target;
             }
             t.apply();
-            recents.onTasksAppeared(targets);
+            mAppearedTargets = targets;
             return true;
         }
 
+        private void commitTasksAppearedIfNeeded(RecentsAnimationListener recents) {
+            if (mAppearedTargets != null) {
+                recents.onTasksAppeared(mAppearedTargets);
+                mAppearedTargets = null;
+            }
+        }
+
         @Override public ThumbnailData screenshotTask(int taskId) {
             try {
                 final TaskSnapshot snapshot =