fix volume and effect enable delay on offloaded tracks

Volume: add a method to wake up the mediaserver playback
thread when a volume command is received on an offloaded track.

Effects: call effect chain process on offloaded playback threads
asynchronously from writes to allow effect state updates while
waiting for async write callback.

Bug: 10796540.

Change-Id: Id2747ae88783575d1d7ffd6fc86fbd054ab2c739
diff --git a/include/media/IAudioTrack.h b/include/media/IAudioTrack.h
index afac4ae..5c8a484 100644
--- a/include/media/IAudioTrack.h
+++ b/include/media/IAudioTrack.h
@@ -90,6 +90,9 @@
 
     /* Return NO_ERROR if timestamp is valid */
     virtual status_t    getTimestamp(AudioTimestamp& timestamp) = 0;
+
+    /* Signal the playback thread for a change in control block */
+    virtual void        signal() = 0;
 };
 
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