AudioTrack: Add getUnderrunCount()

This allows an app to detect application-level output glitches.
Underrun counts survive track recreation.

Change-Id: I8eb14e92f6fc1007718a29b0666ab51ace30cdb8
Bug: 25641253
Signed-off-by: Phil Burk <philburk@google.com>
diff --git a/include/media/AudioTrack.h b/include/media/AudioTrack.h
index fe4611c..602e1f3 100644
--- a/include/media/AudioTrack.h
+++ b/include/media/AudioTrack.h
@@ -305,6 +305,11 @@
      */
             uint32_t    latency() const     { return mLatency; }
 
+    /* Returns the number of application-level buffer underruns
+     * since the AudioTrack was created.
+     */
+            uint32_t    getUnderrunCount() const;
+
     /* getters, see constructors and set() */
 
             audio_stream_type_t streamType() const;
@@ -784,6 +789,8 @@
             // FIXME enum is faster than strcmp() for parameter 'from'
             status_t restoreTrack_l(const char *from);
 
+            uint32_t    getUnderrunCount_l() const;
+
             bool     isOffloaded() const;
             bool     isDirect() const;
             bool     isOffloadedOrDirect() const;
@@ -911,6 +918,8 @@
     bool                    mRetrogradeMotionReported; // reduce log spam
     AudioTimestamp          mPreviousTimestamp;     // used to detect retrograde motion
 
+    uint32_t                mUnderrunCountOffset;   // updated when restoring tracks
+
     audio_output_flags_t    mFlags;
         // const after set(), except for bits AUDIO_OUTPUT_FLAG_FAST and AUDIO_OUTPUT_FLAG_OFFLOAD.
         // mLock must be held to read or write those bits reliably.