Don't send callback with null topActivity

The topActivity defined in ActivityListener#onTopActivityChanged is @NonNull, so GenericWindowPolicyController should not send the callback when topActivity is null.

Bug: 222771130
Test: manual testing
Change-Id: I227d1d83e0e191975ab45b110909938022bb1d0c

core/java/android/companion/virtual/VirtualDeviceManager.java

@@ -469,6 +469,10 @@ public final class VirtualDeviceManager {
         /**
          * Called when the top activity is changed.
          *
+         * <p>Note: When there are no activities running on the virtual display, the
+         * {@link #onDisplayEmpty(int)} will be called. If the value topActivity is cached, it
+         * should be cleared when {@link #onDisplayEmpty(int)} is called.
+         *
          * @param displayId The display ID on which the activity change happened.
          * @param topActivity The component name of the top activity.
          */

services/companion/java/com/android/server/companion/virtual/GenericWindowPolicyController.java

@@ -165,7 +165,10 @@ public class GenericWindowPolicyController extends DisplayWindowPolicyController
 
     @Override
     public void onTopActivityChanged(ComponentName topActivity, int uid) {
-        if (mActivityListener != null) {
+        // Don't send onTopActivityChanged() callback when topActivity is null because it's defined
+        // as @NonNull in ActivityListener interface. Sends onDisplayEmpty() callback instead when
+        // there is no activity running on virtual display.
+        if (mActivityListener != null && topActivity != null) {
             // Post callback on the main thread so it doesn't block activity launching
             mHandler.post(() ->
                     mActivityListener.onTopActivityChanged(Display.INVALID_DISPLAY, topActivity));