Allow changing search bar hint in the cling on-the-fly

- This allows GEL to initially supply an empty string if hotword isn't
  active, and later on update the cling if and when the hotword becomes
  active.
- This isn't expensive as it'll only be called if Launcher first calls
  getFirstRunClingSearchBarHint, i.e. once every fresh install/clear
  data

Bug: 11116105
Change-Id: Ia963bb62d7b331afd0c34adfa6c4b6bdbac48349