Watch for changing enabled IMEs

This CL watches a change to Settings.Secure.ENABLED_INPUT_METHODS and
updates search index of IMEs.

Bug: 32643633
Test: After installing AOSP LatinIME.apk, enabling/disabling the IME
      and searching "AOSP", then verify the search landing page is
      correct depending on the IME's enabling state.
Test: Update robolectric test.

Change-Id: I85fa845238b89375e56b207a014af6432f9db647