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/Tracks.cpp b/services/audioflinger/Tracks.cpp
index e676365..d1ab3c8 100644
--- a/services/audioflinger/Tracks.cpp
+++ b/services/audioflinger/Tracks.cpp
@@ -283,6 +283,11 @@
     return mTrack->setParameters(keyValuePairs);
 }
 
+status_t AudioFlinger::TrackHandle::getTimestamp(AudioTimestamp& timestamp)
+{
+    return INVALID_OPERATION;
+}
+
 status_t AudioFlinger::TrackHandle::onTransact(
     uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
 {