JNI: Prevent duplicate construction of Socket Manager

* There is a race condition in **_AdapterService.cpp where check on
  socket manager pointer and construction of new socket manager instance
  are not protected by a mutex. Thus when called from multiple binder
  threads from the Java layer, multiple socket manager instances could
  be created, causing a crash in StrongPointer assignment
* This CL adds a mutex to protect access to socket manager pointer

Bug: 69621696
Change-Id: Id22b247db6784a51ac922587778634d9b98e87a4
Fixes: 69621696
Test: make, connect to remote device via RFCOMM