Move Session State definition into Session

The state of a session is more closely tied to the session itself,
rather than the EnrichedCallManager. The enum was originally placed in
the Manager as it was used in the API, but now that's no longer the
case.

This CL moves the Session State declaration into Session, which is a
more appropriate location.

Test: tap
PiperOrigin-RevId: 160448612
Change-Id: I907476140bc174a061d48ee8e58878af64a62b29