Scale down thumbnail with app surface

The recents Go app to overview transition has app scale down to a
thumbnail but normally covers the thumbnail.  However, apps with
transparency will be semi-visible and will allow the user to see
the thumbnail in the back at its final size. Instead, we should fit
the thumbnail to the surface so that they both scale down at the same
time.

Bug: 132458092
Test: Go to app with transparency, scale down
Change-Id: Iaebeaaf2ddcfc86fd4f55ef9d8c3f19583947c48