Fix snapshots for dialogs

When taking a snapshot, always include the full stack bounds.

When showing a snapshot starting window, use the SystemUI flags
from the top fullscreen window in the task - this is the same way
as we do it when the real windows are going to be visible.

Test: Revoke contacts permissions, launch contacts, go to recents,
go back.
Test: go/wm-smoke
Fixes: 63268663

Change-Id: If8d9701f6d34abd56116ed04fe27fe82688e506a