[PiP2] Cover draws for non-seamless resize

Cover up the new draws with screenshots after
finishing resizing PiP if the seamless resize
is not enabled. Fade out the screenshot after the
transition is finished.

Bug: 398254469
Flag: com.android.wm.shell.enable_pip2
Test: atest PipSchedulerTest
Test: atest PipTaskListenerTest
Test: pinch to resize a non-seamless resize PiP
Change-Id: If6907344bebc35bb3bab8d4dfc09b61a07ac727e