Pull out commandToString() from FastMixerDumpState.cpp

to FastThreadState.cpp, FastMixerState.cpp, and FastCaptureState.cpp

Change-Id: I872efb211e8a0335c42906367859d3674340e374
diff --git a/services/audioflinger/FastThreadState.h b/services/audioflinger/FastThreadState.h
index 011921d..f18f846 100644
--- a/services/audioflinger/FastThreadState.h
+++ b/services/audioflinger/FastThreadState.h
@@ -46,6 +46,8 @@
     FastThreadDumpState* mDumpState; // if non-NULL, then update dump state periodically
     NBLog::Writer* mNBLogWriter; // non-blocking logger
 
+    // returns NULL if command belongs to a subclass
+    static const char *commandToString(Command command);
 };  // struct FastThreadState
 
 }   // android