Make bouncer background opaque

We keep seeing the bouncer showing over LS content with no scrim
showing.

We cannot continue to rely on the scrim as there has historically been
problems with it being in the right state. The material surface
background was removed from the bouncer due to a weird gray after image
when unlocking. This seems to have been fixed because the bouncer
background and backscrim color are the same color (material surface).

CL for reference:
https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/base/+/24034687

Test: unlock device and observe unlock animation.
Test: swipe down for shade transition and then swipe up immediately.
Test: observe bouncer background is opaque in web-hv
Fixes: 328224320
Fixes: 328548367
Flag: NONE
Change-Id: Ie94195fc65add09faaad4e6c3270e9f05a420e08
(cherry picked from commit 6471f0a7)