Fix screenshot behavior with display cutout

Insets need to be set on the decor view, rather than the screenshot
view, in order to be picked up correctly.

Bug: 179050074
Fix: 179050074
Test: manual; taking a screenshot on a phone with a display cutout
results in a black bar at the top of the screen pre-change and
correctly fills the screen post-change

Change-Id: I4784027009359376624699c4ef3e174c047c086d