[2/n] Create AppCompatSafeRegionPolicy for letterboxing

- A safe region is an area on the WindowContainer in which an activity
  can open without have any occlusion.
- This policy controls how the letterboxing should be applied if a safe
  region is set and required.

Bug: 380132497
Flag: com.android.window.flags.safe_region_letterboxing
Test: atest AppCompatSafeRegionPolicyTests
Change-Id: I2637c80071b295393dc31318e47184c188bcaf20