Fix a2dp state machine java crash

Multiple threads trying to execute the a2dp state machine block,
where the message is sent even before starting the state machine,
resulting in FATAL exception on com.android.bluetooth.

Make the Device State Machine Map synchronized to avoid multiple
threads accessing the same resource at a time. Gaurding
mDeviceStateMap with itself to resolve this synchronization issue.

Bug: 373331233
Flag: EXEMPT, minor bugfix
Test: m com.android.btservices

Change-Id: I5adf470dd623fd674f11526b0f692b9ef1f2cf8a
Signed-off-by: Gowtham Anandha Babu <gowtham.anandha.babu@intel.com>