[1/n] Skip letterboxing for unrelated startingWindow

Added to prevent letterbox surface updates from being applied
to the activity's main window prematurely, while a different starting window
is still active. This ensures letterboxing is correctly managed
for the currently displayed (or soon-to-be displayed) target window.

Flag: com.android.window.flags.app_compat_refactoring_skip_starting_window_letterbox
Bug: 432347028
Test: atest WmTests:AppCompatLetterboxPolicyTest

Change-Id: I910ef122036734fab9a95c68f073f6b3ee47a2dd