audio policy: fix mutex in stopOutput

Commit d7fe086d introduced a problem by acquiring the AudioPolicyService
mutex in stopOutput(). stopOutput() and releaseOtuput() can be called
from audioflinger thread loop and should never acquire the main
AudioPolicyService mutex.
Removed unnecessary check on valid port Id as this check is also performed
when doStopOutput() is executed by the command thread.

Bug: 111966786
Test: verify no timeout when starting or ending a call.
Change-Id: Id09eaae7a4409dfc54c6fbb1eced1624cd7b3536