audioflinger: fix track terminated before playing

When a track is terminated before any audio was written,
we should not send a drain request to the HAL. The drain
will be ignored and we will wait the async callback for ever.

Also fix a potential deadlock by not deleting a track with the
thread mutex held: mPreviousTrack changed from a sp to a raw pointer.

Bug: 11247103.
Change-Id: I0a4b47b099204c7c326d45d700234ab8f72d8eae
diff --git a/services/audioflinger/Threads.h b/services/audioflinger/Threads.h
index 802b784..a5d2169 100644
--- a/services/audioflinger/Threads.h
+++ b/services/audioflinger/Threads.h
@@ -735,7 +735,7 @@
 
     OffloadThread(const sp<AudioFlinger>& audioFlinger, AudioStreamOut* output,
                         audio_io_handle_t id, uint32_t device);
-    virtual                 ~OffloadThread();
+    virtual                 ~OffloadThread() {};
 
 protected:
     // threadLoop snippets
@@ -755,7 +755,7 @@
     bool        mFlushPending;
     size_t      mPausedWriteLength;     // length in bytes of write interrupted by pause
     size_t      mPausedBytesRemaining;  // bytes still waiting in mixbuffer after resume
-    sp<Track>   mPreviousTrack;         // used to detect track switch
+    Track       *mPreviousTrack;         // used to detect track switch
 };
 
 class AsyncCallbackThread : public Thread {