audioflinger: use sp<> for track proxy

AudioTrack and AudioRecord shared memory proxy (Proxy class) is
RefBase so should not be refenced by raw pointer.

Change-Id: Ic63e31cef97503b8e262d5dafd8ce8f1d72d5eb8
diff --git a/services/audioflinger/TrackBase.h b/services/audioflinger/TrackBase.h
index 67a5e58..9463888 100644
--- a/services/audioflinger/TrackBase.h
+++ b/services/audioflinger/TrackBase.h
@@ -158,7 +158,7 @@
     Vector < sp<SyncEvent> >mSyncEvents;
     const IAudioFlinger::track_flags_t mFlags;
     const bool          mIsOut;
-    ServerProxy*        mServerProxy;
+    sp<ServerProxy>     mServerProxy;
     const int           mId;
     sp<NBAIO_Sink>      mTeeSink;
     sp<NBAIO_Source>    mTeeSource;