Fixing a race condition in RecognitionService

Bug #3458256

If an error occurs and simultaneously user cancels the recognition, here is what happens:
1. dispatchCancel() is called since the user requested cancel. It passes the first "if" successfully.

private void dispatchCancel(IRecognitionListener listener) {
       if (mCurrentCallback == null) {
           if (DBG) Log.d(TAG, "cancel called with no preceding startListening - ignoring");
       } else if (mCurrentCallback.mListener.asBinder() != listener.asBinder()) {
           Log.w(TAG, "cancel called by client who did not call startListening - ignoring");
       } else { // the correct state
           RecognitionService.this.onCancel(mCurrentCallback);
           mCurrentCallback = null;
           if (DBG) Log.d(TAG, "canceling - setting mCurrentCallback to null");
       }
   }

2. Error occurs in the app, which sets the mCurrentCallback to null:
       public void error(int error) throws RemoteException {
           mCurrentCallback = null;
           mListener.onError(error);
       }

3. the second "if" is reached in dispatchCancel()
4. boom

Change-Id: I54cdcc98b495d820a2caead1709d8dee968c461e