Synchronize group-related variables

Make sure mDeviceGroupIdMap and mGroupDescriptors
are synchronized by a lock.

Caught NPE when stop() is called right after start() is called.

Bug: 226305090
Bug: 223290604
Tag: #feature
Test: atest BluetoothInstrumentationTests

Change-Id: Ib18a42fe4a06cba1d6e31da6a2fe7dd40be296e3