Correctly implement multi-user theming support

WallpaperColors are now a per user map, and we use UserTracker to
consult the userId. This avoids race conditions, given that all
userIds are coming from the same controller.

Test: manual
Test: atest ThemeOverlayControllerTest
Fixes: 206736090
Change-Id: I1735f483dc8d5913fd223fb5f13c5059e0ece2da