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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