Added synchronized(LOCK) keyword to protect member variables of onCallRemoved and onCallAdded

For conference call, while calls are in transition to merge state, AT+CLCC command is being received from the peer.
The objects in 'mBluetoothCallHashMap' are accessed/modified concurrently from BluetoothInCallService & BluetoothLeCallControl Classes.
This seems to be resulted in 'ConcurrentModificationException' and causes crash.

Bug: 367777221
Test: m packages/modules/Bluetooth with Manual testing.
Flag: EXEMPT
Change-Id: I7723e3f2234c58c472210e00f2a1b5e1f0c47401