Do not allow ScrimView backgrounds

An external agent is setting the background of the ScrimView.
This causes washed out lock screen and AOD flickering.

It's not clear what's doing this so I'm adding a Log.wtf and keeping the
backbround from being set.

PS: also formatted the xml and removed an unused method

Change-Id: I65dce9ce2b097b572c78605e0524b3854f46457f
Fixes: 71527665
Fixes: 68877998
Test: manually add a ripple to packages/SystemUI/res/layout/super_status_bar.xml
(cherry picked from commit c2aa8d2a)