WallpaperColors caching and synchronization

Making sure that colors are being cached in
WallpaperManagerService and that sysui won't
force a new color extraction.

Fixes: 62958267
Test: manual, reboot, look at systrace
Test: runtest -x cts/tests/app/src/android/app/cts/WallpaperManagerTest.java
Change-Id: Ic079a8e3d4d4ad65947b718dcc544f795c16f152