Clear letterbox surface bounds if it is hidden

App may change cutout mode at runtime, then the letterbox
bounds should be cleared if no longer need to show letterbox.
Otherwise, TaskSnapshot uses getLetterboxInsets that always
returns the old result.

Bug: 309593314
Bug: 439555893
Flag: EXEMPT bugfix
Test: Launch an app with windowLayoutInDisplayCutoutMode=never
      And the app updates layoutInDisplayCutoutMode to always.
      Return to home and launch the app again.
      The snapshot starting window should not contain cutout offset.
Change-Id: Ibbef1385a922a6f8f888468020f1d8e2d41d8611