Contacts to display: hide accounts without raw contacts

- Hide extension accounts with no raw contacts.

- Now that building the list hits the database we need to use AsyncTaskLoader.

- Did a little refactoring (e.g. moving mFilter to the adpter) so the code will
  work better with a loader.

Bug 5160471

Change-Id: I780985a73325d4d453962e8ea5dca0d68c3c869e