audio record: fix getPortId() after release

Some race condition can cause AudioRecordingMonitorImpl to call
getPortId() on the client object after it has been released causing
an exception to be thrown by the JNI.

Test: Use audio Recorder app and check that no exception is thrown
when recording stops

Change-Id: I6cc1aee0a2ccae879e2cb5df7af757f33436d1f3