Improve Taskbar touchableRegion debuggability

- Add DebugTouchableRegion which includes the last region that we
  set as well as the reason (code path) for it, which we dump.
- If persist.debug.draw_taskbar_debug_ui is enabled, we will also
  outline the touchable region on the screen.

Flag: None
Test: testThreeButtonsTaskbarBoundsAfterConfigChangeDuringIme
Bug: 315393203

Change-Id: I70b37e8b8804c4a48ffe3956d2a24aa5eab452fd