Add "track default" type to NetworkRequestInfo

The "default network tracking" request is somewhere between a listen
and a request, in previous semantics. It is more like a listen in that
it doesn't cause any network to be brought up, but also like a request
in that will only get callbacks about a single network at a time (not
all networks that might match).

This change minimally splits the NetworkRequest types into 3: LISTEN,
REQUEST, and TRACK_DEFAULT.

Change-Id: I2d113b2f6eed526a7808592bce0118260a56f77c