Add support for multi-endpoint.

- Add new "ExternalCallNotifier" which tracks external calls that Telecom
informs Incall about.
- Refactored some common code from StatusBarNotifier that is used in
ExternalCallNotifier into NotificationUtil so it can be used in both
places.
- Modified CallList to track and store external calls seperately from
regular ones.
- Added support for triggering a call pull in the
NotificationBroadcastReceiver.

Bug: 27458894
Change-Id: I505c8b5f7aad273ebdaaeae2431564d10b23770b