Conference call merge button visibility issue. (2/2)

- Adding onConferenceableCallsChanged override in Incall Call object.
This ensures that changes to the conferenceable calls list for the
active call triggers UI updates as required.
- Added onDetailsChanged listener in CallButtonPresenter; this ensures
that changes to the call which occur without it changing status are
reflected in the UI.

Bug: 17767618
Change-Id: I6c6175c0bab46989ddfc4c4d8f2087e527febbeb