[Call Timing] Add constants for call timing extras

This CL defines new extras that can be set in a call's intent extras.
These extras track the following:
    CALL_CREATED_TIME_MILLIS - the time when the call was created by
	telephony (or another connection service)
    CALL_TELECOM_ROUTING_START_TIME_MILLIS - the time when telecom
	started processing the call
    CALL_TELECOM_ROUTING_END_TIME_MILLIS - the time when telecom
	finished processing things like call blocking and was ready to
	connect to the UI

These extras can be used by the dialer to track how long it takes for
calls to be shown to the user.

Bug: 28202119
Change-Id: I8fca259d449adedaeb4ff91d35bf59a7409be866