Offset ghost views by the containing view's location

Shared transitions weren't behaving correctly in some cases on
phones with display cutouts, because the locations of the shared
views are computed based on screen coordinates, but take place in
a decor view that may be inset by the display cutout (depending
on the target app's behavior around the cutout).

This change offsets the ghost views according to the location of
the decor view in the screen, so that all adjustments take place
in the same coordinate system.

Bug: 190757843
Test: manual (tested in screenshots, when the target app does and
does not get inset by the cutout)

Change-Id: Ia9c3b5f36136c8825c67ce645f6cbdd8e0126fdf