More cleanly separate requests and listens.

Currently, NetworkRequest has several types of requests (LISTEN,
TRACK_DEFAULT, REQUEST, BACKGROUND_REQUEST), and we expect more
to be added.

There are really three categories of request:
1. Requests satisfied by only one network and will keep that
   network up, and thus need to be sent to NetworkProviders:
   REQUEST, BACKGROUND_REQUEST.
2. Requests satisfied by only one network but will not keep that
   network up: TRACK_DEFAULT
3. Requests satisfied by multiple networks and will not keep any
   networks up: LISTEN.

Unfortunately the separation is not very clear. Currently, for
any valid request, either isListen() will return true or
isRequest() will return true. This makes it impossible to tell
whether a particular request should be sent to NetworkProviders,
so the current code sends TRACK_DEFAULT requests to
NetworkProviders as well. This is incorrect - a TRACK_DEFAULT
should never keep a network up, for example.

This CL attempts to clarify things by making isRequest() return
false for TRACK_DEFAULT requests and thus never sending them to
NetworkProviders. After this CL:

- isRequest will return true only for requests that attempt to
  bring up or keep up a network.
- isListen will return true only for requests that match multiple
  networks but do not keep any of them up.
- Neither will return true for TRACK_DEFAULT.

Test: atest ConnectivityServiceTest
Change-Id: I7aad30ade8f7ab2a179e53483d9afd8675f64a12