Ignore letterbox insets for back animation target.

All animation will play the animation target with it's letterbox, so
there can ignore that insets, in case of shell being confused when
handling taskbar insets.

Bug: 328707998
Test: launch app with a bottom letterbox, verify letterbox area won't
be clip when back-to-home.

Change-Id: I195f437c04cc5dd2abe19c7f4f888dd3888296e2