Don't show button if fullscreen override is from system

If the fullscreen override is from system and not from user choice, we
should not show the user aspect ratio button. Button should still show
persistently if system fullscreen override is not enabled and user has
overridden app to fullscreen.

Bug: 310816437
Test: atest LetterboxUiControllerTest
Change-Id: Id8e5c3fe28a92e775cb137937faba9c44b6e32eb