AudioRecord: Fix uninitialized use of identity

Remove the unnecessary member variable.

Test: adb shell dumpsys media.metrics --all | grep record | grep _allowUid
Test: atest AudioRecordAppOpTest
Bug: 183659794
Change-Id: I2b1eb248b157265fd9847bd4f577da627ebb5036
diff --git a/services/audioflinger/PlaybackTracks.h b/services/audioflinger/PlaybackTracks.h
index ca9b747..2e59baa 100644
--- a/services/audioflinger/PlaybackTracks.h
+++ b/services/audioflinger/PlaybackTracks.h
@@ -336,7 +336,7 @@
                                 audio_format_t format,
                                 audio_channel_mask_t channelMask,
                                 size_t frameCount,
-                                android::media::permission::Identity& identity);
+                                const android::media::permission::Identity& identity);
     virtual             ~OutputTrack();
 
     virtual status_t    start(AudioSystem::sync_event_t event =