Add API support for multi-endpoint.

This CL includes changes required to support multi-endpoint (see the
design doc linked off the bug).

Main changes include:
- support for indicating if a call/connection is "external" to the device.
- support for indicating if an external call can be pulled from the
remote device to the local device.
- API used to initiate a pull of a call external to the current device to
the current device.
- Made the "connection event" API public (was previously @hide); this will
be used to support passing some error events involving pulling calls to
the incall ui.
- Added new InCallService metadata which will be used to determine if an
InCallService wants to be informed of external calls.
- New disconnect causes which will be used to expose the fact that a
multi-endpoint call was answered elsewhere and that a call ended because
it was pulled to another device.
- New call log call types to indicate if calls were answered elsewhere
or pulled to another device.

Bug: 27458894
Change-Id: I423f64ff965b5e50194635a51868c327782db2a1