Make ForegroundCoordinator single-threaded

The appops callback is frequently called from a background thread, which
was causing a crash when calling mNotifPipeline.getActiveNotifs().
Switched the class over to being exclusively single-threaded; it didn't
need to support multiple threads and this simplifies things.

Test: atest SystemUITests:ForegroundCoordinatorTest
Change-Id: I4f010c16fa3f2467a5aaa05ef72c0278396a30e0