Skip letterboxing if the activity below is embedded

Embedded activities are resize-able and won't be letterboxed

Bug: 270681405
fixes: 280898266

Test: SizeCompatTests
Test: manual - repro step in bug

Merged-In: Ic7b1c4b40960fd04de9efbf4f6d7abee45c93025
Change-Id: Ic7b1c4b40960fd04de9efbf4f6d7abee45c93025
(cherry picked from commit b828087b)