Add Connection.startActivityFromInCall

This CL adds a new public API to allow Connections
to start an activity on top of the in-call UI.

The Connection passes a PendingIntent to Telecomm which
then forwards it on to InCallUI. The Connection can cancel
the operation by calling PendingIntent.cancel().

This allows services like SIP and Hangouts to show dialogs
to the user when making a call.

Change-Id: I65119a89c925a93467d1b27304ffec9b088b172f