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

(cherry picked from commit b828087b)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ad7d89c22bf53fbcfd1f8d5a1ae2c611c376e9b3)
Merged-In: Ic7b1c4b40960fd04de9efbf4f6d7abee45c93025
Change-Id: Ic7b1c4b40960fd04de9efbf4f6d7abee45c93025