Add functions to update on conference changes.

+ Add onConferenceChanged() to Connection, so that the Connection can
make act on changes to the conference.
+ Add a utility function to PhoneCapabilities to remove a capability
from a PhoneCapabilities bitmask. Figure this abstracts some bit
logic away to make things more readable.

Bug: 17429707
Change-Id: I0c97731a95dd6573488cd10f094a63abe2428bce