Finalize MEP functionality.

1) Finish plumbing of PULLING_CALL state.
2) Add new disconnect cause used when maximum number of calls across
all devices has been reached.
3) Add PII mask for toString of ImsExternalCallState.

Bug: 29522023
Change-Id: I78a0a9f3c3d846cfc58a1c5bcc6f105027602cbc

telecomm/java/android/telecom/Connection.java

@@ -1551,6 +1551,8 @@ public abstract class Connection extends Conferenceable {
                 return "RINGING";
             case STATE_DIALING:
                 return "DIALING";
+            case STATE_PULLING_CALL:
+                return "PULLING_CALL";
             case STATE_ACTIVE:
                 return "ACTIVE";
             case STATE_HOLDING: