audio layer: prevent a deadlock

A deadlock is caused by a lock-order-inversion of the private
mutex from pulse-audio thread loop, and the callMutex_ mutex
from the Call class. It occurs when concurrently calling
writeToSpeaker() and onNegoDone() callbacks.
The deadlock is prevented by avoid to a access Call data from
the pulse-audio thread.

Gitlab: #623

Change-Id: I91d936b37528db3de24e93b0d49d686f5ca11813