Add a background NetworkRequest type for mobile data always on.

Like a normal (foreground) request, a background request is only
satisfied by one network and will keep that network up. Unlike a
foreground request, when a network only has background requests,
it will linger, and after lingering is complete, it will become
a background network.

Future CLs will cause the system to treat background networks
differently, e.g., by requiring different permissions.

Bug: 23113288
Change-Id: I40f735269dad1042eb04fea15e64584fc903ccb3