alsa_sound: Fix mExtOutMutex related lock issue

-extoutThread was taken slightly longer than on MR1.
-On newer platform, It takes more time to write becausea2dp_write
is a blocking call and takes about 20 ms in every execution
-Because of this, the new stream that was coming in was not getting
the lock.
-To solve this, added a new lock that we acquire before write, and
this lock is taken only during A2DP disconnect since that is the only
case when we close the extoutput

Change-Id: I4ce0bcb17ba2e9fbd60aacd2315d92808509d150