Use a Region when calculating TouchableRegion for cutouts

HeadsUpManagerPhone handles adding extra touchable region for the
display cutout. When this changed to use Rects, the extra touch region
was applied to the entire phone width because Rect#union() will cause
the resulting Rect to grow in every dimension. The fix is to use a
Region instead so we can properly union only the space under the notch

Fixes: 135487528
Test: manual
Change-Id: I1d39785087cc3378fc42363df170b81b84420a8e
(cherry picked from commit e1822fd8)