Bluetooth: Adding synchronization to handle multiple threads

If binder dies in the middle while OnserviceDisconnected is
being processed,there is a chance that other thread through
onbinderdied will also enter to onServiceDisconnected before
service is moved to NULL. Extending the synchronization to
OnServiceConnected to avoid simultaneous calls from addproxy
and removerproxy in corner cases which gets called from
different thread context.

Change-Id: Ib0e2f97ea1b98afc4e83dcfc1df21577485f86ca