Prevent Person duplication from peer vCards

Due to the ability to receive vCards from other peers (during a
Call or from a ContactRequest), there are many possibilities to
create duplicate Person objects. This is because up until now we
have been using the Person's UID as a unique identifier. However
the UID has come from the vCard, and we cannot trust the vCards we
receive from peers to have either unique UIDs when they are
different contacts, nor the same UID if its the same contact.

Thus in the case of received vCards, we identify whether or not
a new Person should be created based on the ContactMethod.

Note that this doesn't necessarily solve the issue in the case
we have multiple ContactMethods with the same RingID (this may
happen when we have multiple Accounts communicating with the same
peer). However, this is a step in the right direction.

Change-Id: I9c05fe00fc46ba0e26e8886cad33cf0fd4afeb81