[7/n] Add application and activity property to allow safe region letterboxing

- This new property can be used by activities or applications to allow safe region letterboxing.
- Once set, the platform will not override this behavior.
- Not setting this property at all, or setting this property to true has no effect.
- Setting this property to false will not allow the platform to
  letterbox the activity or application within a safe region.

Bug: 380132497
Bug: 368408934
Flag: com.android.window.flags.safe_region_letterboxing
Test: atest SizeCompatTests
Change-Id: I5835ee3a7b55ffdaeb696c3ee9ae70d658632947