Avoid running RegisteredServicesCache tasks on main thread

Since RegisteredServicesCache tasks from SyncManager and AccountManager
run on the main thread of SystemServer, It could cause draining
the main thread and end up with ANR when receiving a lot of broadcasts
or notifying changes through a listener.

This CL changes a few things to avoid running it on main thread.
1. RegisteredServicesCache uses BackgroundThread by default for
receiving broadcasts and a listener, instead of main thread.
2. AccountManager uses its own handler for a listener.

Bug: 171907687
Test: atest SyncManagerTest CtsSyncManagerTest AccountManagerServiceTest
Change-Id: I838c15b797b46fd58bb3c6ce9ee444a4737775d8