Base APN State on the state of the ApnContext.

Stop using enablement or activeness as a proxy for state and
just ask for the state.  Enablement has changed (since L)
to indicate if we should look for a connection and is not
representative of if we have a connection.

bug:26785653
Change-Id: I8c1edda733509d2539989e727809c13002abf37f