Dynamically add AnswerFragment

AnswerPresenter is a little tricky because it is in charge of
responding to incoming calls, so we want it to be alive and getting
onIncomingCall callbacks, but we don't want to create an instance
of AnswerFragment, which it is dependent on.

Move some dependencies around so that AnswerPresenter is created
by InCallPresenter instead. It registers for callbacks via InCallPresenter
instead of directly to CallList, and shows/hides the AnswerFragment via
InCallActivity.

Change-Id: I7026150988bf3cda762dda8a319f48e1af132361