Clean up corpus update handling

This fixes two problems:
- makes the Voice Search button in the search widget and the QSB app
  update immediately when Voice Search is installed or uninstalled.
- updates the corpus selection dialog and the currently selected
  corpus correctly when corpora are installed or uninstalled.

Bug: http://b/issue?id=2522135
Change-Id: I04b212763e922cb2a9b2542b30ec4091a7dea506
12 files changed