ConnectionService API has only one completed callback (2/3)

Refactor ConnectionService API so it has only one "completed"
callback, and connection state and failure codes indicates what
happened. Previous design where we had separate callbacks for failure,
cancellation and success was error prone because it was easy to forget
to implement one of them.

This particular change in this set of changes also makes one crucial
fix. The Call object, when it is notified of an unsuccessful attempt
to make a connection, previously told itself:

  setState(CallState.DISCONNECTED);

But that led to its DISCONNECTED state never being published to
anyone, while on the other hand it was now removed from the
CallIdMapper, so nobody could talk to it (symptom: InCall UI hangup
button becomes a no-op). The correct thing to do is:

  CallsManager.getInstance().
      markCallAsDisconnected(this, code, msg);

which goes through the CallsManager to set the appropriate states. I
think that, post-L, our code here is really ripe for a refactoring
that prioritizes encapsulation and more ubiquitous use of the
"listener" pattern, otherwise we're going to have a bunch more of
these kinds of bugs. :)

Bug: 16993846
Bug: 17070939

Change-Id: Ie79e585dfbc2a1b79a3721d749855704cd8270ed