Restore to split for request fullscreen API

This change does the following:
1. Saves the parent of a split-screen task when it is requesting
   fullscreen using the request fullscreen API.
2. When task is requesting to restore to its original windowing mode,
   the parent will also be restored so that the StageCoordinator can
   intercept the transition and animate the task to fullscreen.

Test: Restoring to split using request fullscreen API
Bug: 315352482

Change-Id: I6dfd34b237cadbb9599e6441c77bceee9f340554