Letterbox positioning (2/4): Rename "task letterbox" to "fixed orientation letterbox".

This is a follow up for ag/13575049 which moved task level letterbox to activity level.

Also, change order of aspect ratio overrides in resolveFixedOrientationConfiguration to ensure that aspect ratio restrictions for the activity will be respected and adding corresponding tests to prevent regressions.

Bug: 175212232
Test: atest SizeCompatTests
Change-Id: Ib911731781895495760001a1f9af1e8d37b1ded4