fix regression RecordingActivityMonitor

commit c7a0cdfe introduced a regression in audio recording
activity monitoring by using the wrong key to lookup for a client in
active recordings when a session stops.

Bug: 122113404
Test: android.media.cts.AudioRecordingConfigurationTest#testAudioManagerGetActiveRecordConfigurations
Change-Id: I06d87d086f391ae6e9adcd88f2235e002f88e682