Settings: Alert user when they attempt to add an invalid number to BL.

  - BlacklistProvider will reject numbers that aren't valid, silently. So instead of
  resuming to a blank screen, we should alert the user that the number they attempted to
  add is invalid by utilizing BlacklistUtils#addOrUpdate method.

Change-Id: I1ca32427baad41c1c9379e18046e6ca7937b6d32