Add IAudioTrack::getTimestamp()

with dummy implementation in AudioFlinger::TrackHandle, and
implement AudioTrack::getTimestamp() using IAudioTrack.

Also document invariant that mAudioTrack and control block are always
non-0 after successful initialization.

Change-Id: I9861d1454cff7decf795d5d5898ac7999a9f3b7e
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index e5e4113..5df04f4 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -411,6 +411,7 @@
         virtual status_t    setMediaTimeTransform(const LinearTransform& xform,
                                                   int target);
         virtual status_t    setParameters(const String8& keyValuePairs);
+        virtual status_t    getTimestamp(AudioTimestamp& timestamp);
 
         virtual status_t onTransact(
             uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags);