Fix black holes and flickering in docked resizing

When we start a resize with the docked stack divider,
set the surface background to be full-screen, and use
the traditional surface clipping/positioning in window
manager to adjust the size. This ensures that we don't
have any black holes because of asynchronicity (except
at the very beginning, but this can be worked around
later), and the position of the right/bottom activity
is always in sync with the position of the divider.

Also fix a bug in NonClientDecorView where the first
request to draw was dropped (because the thread hasn't
started up yet), and the main thread was waiting for it
indefinitily.

Bug: 24507122
Change-Id: I623bd48d5be64fac2fba45241b84f265944d200d