Remove stopped activity from UnknownAppVisibilityController.

UnknownAppVisibilityController tracks activities whose visibility are
not yet settled, and delays app transition. When activity is stopped,
it doesn't appear on the screen anymoere, so we shouuld remove it from
UnknownAppVisibilityController.

Bug: 191392295
Test: manual
  1. go to lock screen
  2. tap calender
  3. check if the calender app starts within a couple of seconds.
Change-Id: I2c99469d44ee6c1e3ccd7b7b915321ecce0900ce

services/core/java/com/android/server/wm/ActivityRecord.java

@@ -5015,6 +5015,7 @@ final class ActivityRecord extends WindowToken implements WindowManagerService.A
         mAppStopped = true;
         // Reset the last saved PiP snap fraction on app stop.
         mDisplayContent.mPinnedTaskController.onActivityHidden(mActivityComponent);
+        mDisplayContent.mUnknownAppVisibilityController.appRemovedOrHidden(this);
         destroySurfaces();
         // Remove any starting window that was added for this app if they are still around.
         removeStartingWindow();