Prevent merging conference calls hosted on peer device.

With IMS, the "multiparty" bit on an ImsCall is set to "true" when a call
is merged into a conference.  This not only occurs on the device hosting
the conference call, but also on the devices of the callers merged into
the conference.

This CL adds code to propagate changes in the multiparty state of a call
from the ImsCall up to the ImsPhoneConnection.

Bug: 19478784
Change-Id: I51b84731dbc77a235d2602db751af016e6c397ea