Permit system dialer callscreeningservice to access sip invite.

Similar to how the system dialer can use Connection#EXTRA_SIP_INVITE,
adding the ability for the callscreeningservice which is implemented by
the system dialer to also access this extra key value.

Test: Added new unit tests
Bug: 132122961
Change-Id: I402c8c0bfd39322820c65787e24f2f6df842e7c7