Fix swipe-to-home not enter pip from split properly

Unlick 3-button home-key to enter pip from split flow, if using gesture
navigation, the flow is like:
  swipe to home -> handle as a TYPE_RECENTS_DURING_SPLIT ->
  finishTransition checks and triggers auto-pip -> create TRANSIT_PIP ->
  reparent task to TDA -> collect to new transition -> file TRANSIT_PIP
Since the task was collected after reparent, shell-side unable to
identify if it was in split when receiving TRANSIT_PIP.
This checks if a TRANSIT_PIP implies empty one side of the split, so we
can handle enter-pip-from-split properly.

Fix: 287002454
Test: 1. Return to home from a split pair, the activity on the
         split can enter PiP and clear the split pair in overview.
      2. Put a split pair in background, launch another
         fullscreen activity and request entering PiP, the log
         should not show:
         "Got a PiP-enter request while Split-Screen is active".
Change-Id: Ifc129e85a69e7c05363204e72fe4cc6ba61b9b47

libs/WindowManager/Shell/src/com/android/wm/shell/splitscreen/StageCoordinator.java

@@ -364,6 +364,27 @@ public class StageCoordinator implements SplitLayout.SplitLayoutHandler,
         return mMainStage.isActive();
     }
 
+    /** @return whether the transition-request implies entering pip from split. */
+    public boolean requestImpliesSplitToPip(TransitionRequestInfo request) {
+        if (!isSplitActive() || !mMixedHandler.requestHasPipEnter(request)) {
+            return false;
+        }
+
+        if (request.getTriggerTask() != null && getSplitPosition(
+                request.getTriggerTask().taskId) != SPLIT_POSITION_UNDEFINED) {
+            return true;
+        }
+
+        // If one of the splitting tasks support auto-pip, wm-core might reparent the task to TDA
+        // and file a TRANSIT_PIP transition when finishing transitions.
+        // @see com.android.server.wm.RootWindowContainer#moveActivityToPinnedRootTask
+        if (mMainStage.getChildCount() == 0 || mSideStage.getChildCount() == 0) {
+            return true;
+        }
+
+        return false;
+    }
+
     /** Checks if `transition` is a pending enter-split transition. */
     public boolean isPendingEnter(IBinder transition) {
         return mSplitTransitions.isPendingEnter(transition);

libs/WindowManager/Shell/src/com/android/wm/shell/transition/DefaultMixedHandler.java

@@ -24,7 +24,6 @@ import static android.view.WindowManager.TRANSIT_TO_BACK;
 import static android.window.TransitionInfo.FLAG_IS_WALLPAPER;
 
 import static com.android.wm.shell.common.split.SplitScreenConstants.FLAG_IS_DIVIDER_BAR;
-import static com.android.wm.shell.common.split.SplitScreenConstants.SPLIT_POSITION_UNDEFINED;
 import static com.android.wm.shell.splitscreen.SplitScreen.STAGE_TYPE_UNDEFINED;
 import static com.android.wm.shell.splitscreen.SplitScreenController.EXIT_REASON_CHILD_TASK_ENTER_PIP;
 import static com.android.wm.shell.util.TransitionUtil.isOpeningType;
@@ -33,7 +32,6 @@ import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.app.PendingIntent;
 import android.os.IBinder;
-import android.util.Log;
 import android.util.Pair;
 import android.view.SurfaceControl;
 import android.view.WindowManager;
@@ -161,9 +159,7 @@ public class DefaultMixedHandler implements Transitions.TransitionHandler,
     @Override
     public WindowContainerTransaction handleRequest(@NonNull IBinder transition,
             @NonNull TransitionRequestInfo request) {
-        if (mPipHandler.requestHasPipEnter(request) && mSplitHandler.isSplitActive()
-                && request.getTriggerTask() != null && mSplitHandler.getSplitItemPosition(
-                        request.getTriggerTask().token) != SPLIT_POSITION_UNDEFINED) {
+        if (mSplitHandler.requestImpliesSplitToPip(request)) {
             ProtoLog.v(ShellProtoLogGroup.WM_SHELL_TRANSITIONS, " Got a PiP-enter request while "
                     + "Split-Screen is active, so treat it as Mixed.");
             if (request.getRemoteTransition() != null) {
@@ -606,6 +602,11 @@ public class DefaultMixedHandler implements Transitions.TransitionHandler,
         return false;
     }
 
+    /** @return whether the transition-request represents a pip-entry. */
+    public boolean requestHasPipEnter(TransitionRequestInfo request) {
+        return mPipHandler.requestHasPipEnter(request);
+    }
+
     @Override
     public void mergeAnimation(@NonNull IBinder transition, @NonNull TransitionInfo info,
             @NonNull SurfaceControl.Transaction t, @NonNull IBinder mergeTarget,