Define NOT_ROAMING network capability.

The "roaming" state of a network really belongs on NetworkCapabilities
instead of being published through NetworkInfo.isRoaming().  One major
reason is to support developers creating NetworkRequests for a
non-roaming network.

This change unifies NetworkInfo updates into a single method, since
there were lurking bugs where it could end up partially updated.  Also
always send updated info and capabilities whenever they change.

Test: bit FrameworksNetTests:android.net.,com.android.server.net.
Bug: 68397798, 16207332
Change-Id: I1652e7bea37e1d3f64ef829a9cbc1411b64e286c