Add extras to Connections/Calls. (2/3)

Two major changes:
1) Add the notion of extras to a Connection.  These extras will be
parceled through to InCallService as Call.getExtras()
2) The previously existing Call.getExtras() has been renamed to
getIntentExtras(). This name better describes the fact that these
particular extras are from the original CALL or INCOMING_CALL intents.

Change-Id: I4eed1e1bb47e83f8ba5c843cab7e7e492a297c71