Attemp to fix letterbox flicker before app window draw.

While starting app & before app window draw, letterbox can only check
whether to show letterbox by starting window, but starting window can
transfer to another activity during start app process.
To ensure letterbox state stable during animation, only check the
visibility state of activity, so it won't affected by window.

Bug: 328598627
Test: atest SizeCompatTests LetterboxUiControllerTest
Test: launch app throught trampoline activity, verify the letterbox of
closing activity stay visible during transition animation.

Change-Id: I6c18c9b507f0c732d58c739bee068f835fad4391
Merged-In: I6c18c9b507f0c732d58c739bee068f835fad4391