Prevent refreshing app search results with empty query.

- Any change to the TextView text will cause mQuery to be set,
  which will cause a new search next time refreshSearchResult
  is called.  We should also be checking there if it is a valid
  search query before starting a new search.

Bug: 30606307
Change-Id: I08640c56199211f2aeea2386fcf699810853ab58