Take latency and current time into account for visualization

Buffer more data, and return the data that is currently being
output from the audio output, to ensure that visualizations are
smooth and responsive even when the audio output has a large
latency and/or large buffers.
b/6522728

Change-Id: I401637f01be7600b3c594a55c869036c13b206c0
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index a0e0ea5..1c44f2f 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -965,6 +965,8 @@
 
                     // return estimated latency in milliseconds, as reported by HAL
                     uint32_t    latency() const;
+                    // same, but lock must already be held
+                    uint32_t    latency_l() const;
 
                     void        setMasterVolume(float value);
                     void        setMasterMute(bool muted);