ScreenDecor: Fix cutout overlay regression

Fixes an issue with the cutout software overlay (for anti-aliasing and emulation)
that only became apparent once the assist hints were added to the overlay; the cutout
view did not set the appropriate gravity, which caused it to be aligned wrongly in certain
orientations, or with bottom cutouts, now that the window got bigger due to the assist hints.

Fixes: 134052901
Test: Enable double cutout, verify that the emulated cutout is not cut off at the bottom
Change-Id: Ied19c117a9ee560310ce783633f001a6003de869