Call AudioTrack.flush() before AudioTrack.release().

On NVIDIA Shield, recreated passthrough AudioTracks have incorrect playback head
positions, due to shared state with the previously destroyed passthrough
AudioTrack. Calling AudioTrack.flush() before AudioTrack.release() ensures this
state is cleared.

This is a safe change so I have not made it a device-specific workaround. The
combined time for flush and release is less than 10 ms (with flush normally
taking less than two ms).

library/src/main/java/com/google/android/exoplayer/audio/AudioTrack.java

@@ -611,6 +611,7 @@ public final class AudioTrack {
         @Override
         public void run() {
           try {
+            toRelease.flush();
             toRelease.release();
           } finally {
             releasingConditionVariable.open();