audioflinger: fix invalidateTracks() on offload thread

Commit 05317d29 introduced a regression causing a systematic flush
on offload output thread when invalidateTracks() is called even if
no track is using the stream type specified.

Bug: 28786996
Change-Id: Iee393ee8b1093c40647235bd35f4a6fa9b62937a
diff --git a/services/audioflinger/Threads.h b/services/audioflinger/Threads.h
index 1cceb6d..787b5c4 100644
--- a/services/audioflinger/Threads.h
+++ b/services/audioflinger/Threads.h
@@ -613,7 +613,7 @@
                 virtual bool     isValidSyncEvent(const sp<SyncEvent>& event) const;
 
                 // called with AudioFlinger lock held
-                        void     invalidateTracks_l(audio_stream_type_t streamType);
+                        bool     invalidateTracks_l(audio_stream_type_t streamType);
                 virtual void     invalidateTracks(audio_stream_type_t streamType);
 
     virtual     size_t      frameCount() const { return mNormalFrameCount; }