Only unregister USB receiver if it was registered

This will probably prevent a crash on pre-Honeycomb devices.

android/src/com/levien/synthesizer/android/ui/PianoActivity2.java

@@ -189,7 +189,9 @@ public class PianoActivity2 extends Activity {
   protected void onPause() {
     Log.d("synth", "activity onPause");
     androidGlue_.setPlayState(false);
+    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB_MR1) {
       unregisterReceiver(usbReceiver_);
+    }
     setMidiInterface(null, null);
     super.onPause();
   }