Mark screen decorations as trusted overlays

These decorations set flag_slippery, to cover the unlikely event of user
tapping a screen decor and then not able to send this touch gesture to
the neighboring window.

Bug: 157929241
Test: `adb shell dumpsys input` and make sure that ScreenDecorOverlay
and ScreeDecorOverlayBottom have FLAG_SLIPPERY

Change-Id: Ib276764edcf1acffe94e3db7397fd23aaaa5dbaa