Add a missing @Override to IMMS#getVrInputMethodList()

This is a small follow up CL to a previous CL [1] that added
InputMethodManagerService#getVrInputMethodList() but forgot to add
@Override annotation there.

 [1]: I1db7981b5198e7e203d4578cae7e5b6d20037d0d
      89a6c48a

Bug: 63037786
Test: compile
Change-Id: I9371f70f48a19efa7cff3a31be80dfa987e02d8c