Introduce Callable URI as hidden API

New Phone UI wants both phone numbers and Sip addresses, which are
all "callable" data. This API allows ContactsProvider to return
both as one unified Cursor.

There are a few caveats:

- There's no MIMETYPE for callable, since it is not an actual data
  type.
- Each row may require different interpretation for each column.
  For example, the meaning of type (data2) would be very different
  between Phone and Sip. Caller applications must absorb those minor
  differences by themselves (at least for now).

Bug: 5039627
Change-Id: I4729c87ecd11296599ed0313e8ad1f5aeabbd32b