Letterbox: show activity under insets when possible or needed

Activity handles insets on its own (e.g. under status bar or navigation bar). After task's surface is repositioned, intersection between an activity and insets can change but if it doesn't, the activity should be shown under insets to maximize visible area.

Also, an activity can use area under insets and insets shouldn't be cropped in this case regardless of a position on the screen.

Test: atest LetterboxTaskListenerTest
Fix: 174086576
Change-Id: I84cd28520ead3fb0099f15cc672167871c7d78a1