Move SyncManager broadcast receivers off the main thread

Use the SyncManager handler instead of main thread to
avoid blocking the main thread on locks from other services.

Also avoid locking repeatedly to check if jobscheduler is
initialized.

Bug: 409177829
Flag: com.android.server.am.syncmanager_off_main_thread
Test: atest CtsSyncManagerTestsCases
Change-Id: Ia12f8d006e1d9c3bdb221650506c652cd245d4aa