Make the MulitSelect contact ID column configurable

* So that the MultiSelectAdapter can support different kinds of contact
  lists (e.g. aggregate contacts or raw contacts).
* Also move utility method to convert a MultiSelected contact IDs TreeSet
  into an array to the MultiSelectAdapter class since more than just
  the PeopleActivity will need to do it when there are more MulitSelect
  lists.

Bug 18641067

Change-Id: I136137b3f7d3c7a7faa0f381eb66d9648a46cf5c