AudioFlinger: Accumulate long term thread latency statistics.

This results in a more stable value.

Test: AudioFlinger dumpsys
Change-Id: I72f5e819076502328222b3bf6bdc761b8aad4a87
diff --git a/services/audioflinger/Threads.h b/services/audioflinger/Threads.h
index 4968829..1afea08 100644
--- a/services/audioflinger/Threads.h
+++ b/services/audioflinger/Threads.h
@@ -520,6 +520,7 @@
                 // This should be read under ThreadBase lock (if not on the threadLoop thread).
                 audio_utils::Statistics<double> mIoJitterMs{0.995 /* alpha */};
                 audio_utils::Statistics<double> mProcessTimeMs{0.995 /* alpha */};
+                audio_utils::Statistics<double> mLatencyMs{0.995 /* alpha */};
 
                 bool                    mIsMsdDevice = false;
                 // A condition that must be evaluated by the thread loop has changed and