SystemServer: Broadcast callback no longer need synchro

RemoteCallbackList implements synchronization which enables calling register / unregister as a broadcast is active.

It is still invalid to start two concurrent broadcasts, but this possibility is prevented by the fact that broadcasts are now all run on the same thread.

Bug: 288450479
Test: m Bluetooth
Flag: Exempt refactor cleanup
Change-Id: I505d61049eb5b643d3e993dd5ddd2404b3f6cf9a