Finalizing Multiendpoint functionality.

1) Finish plumbing of PULLING call state.
2) b/29522023 - Handle case where the maximum number of calls across all
devices has been reached and a new call cannot be placed; report a new
Telephony disconnect cause which maps to an appropriate user friendly
message.
3) b/29633039 - Make the ImsExternalCallTracker keep track of the last
call pull state for each external call.  Also, track whether the device is
in a call, and whether video is enabled on the device.  Use this to enforce
the rules that an external video call cannot be pulled if video is not
enabled on the device, and that calls cannot be pulled when there is an
active call on the device.
4) Fixed a circular initialization dependency between ImsPhoneCallTracker
and ImsExternalCallTracker.  Previously, the ImsPhoneCallTracker would
spool up the ImsMultiEndpoint service and set the ImsExternalCallTracker
as a listener.  HOWEVER, the ImsEXternalCallTracker isn't actually
created until AFTER the ImsPhoneCallTracker is.  It just so happens the
ImsPhoneCallTracker init happens on a thread, so ImsExternalCallTracker
would normally be initialized before ImsPhoneCallTracker.  Whoops.

Bug: 29522023
Bug: 29633039
Change-Id: Iddabb61dcad79c93665c28e07b333e807cbc576d