Color extraction should not block switchUser

switchUser is called from the main thread. This would block it
until color extraction ends, possibly causing a timeout in
ActivityManager.

We're now offloading color extraction to another thread.

Test: Change wallpapers, switch users.
Change-Id: I570c3ce5a562b56106d614c8febc12134e151afc
Fixes: 65146279
1 file changed