[2/n] Remove flag on startingWindow letterboxing check

The fix is small and it's restoring existing and well tested code
making the flag not necessary.

Flag: EXEMPT small bug fix
Bug: 432347028
Test: atest WmTests:AppCompatLetterboxPolicyTest

Change-Id: I895f54c32f197e413c0a0d91c0de8d0b74b1cbed