Override fillsParentBounds in ActivityRecord

- Regression from ag/32872149, since letterboxed activities may not fill the entire parent bounds, it would return false and therefore all sibling activities would still remain visible and paused.
- Override method in ActivityRecord since the activity will always fill parent bounds.

Bug: 413114394
Test: manual, open letterboxed activities and verify WM visibility states.
Flag: com.android.window.flags.enable_see_through_task_fragments
Change-Id: Iba4d9f7b2738907475bbbc0c463588e3d5bd3a8b