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.

Watch for any capability changes that network statistics are
interested in (either metered or roaming) and notify it to perform
an update pass; fixes bug where we previously only triggered on
roaming changes.

Fix bug in VPNs where metered/roaming capabilities of underlying
networks weren't being propagated; this was probably preventing
some jobs from running over unmetered networks, and causing other
jobs to run over roaming networks!  Also passes along link bandwidth
information from underlying networks, and propegates any changes
to underlying networks.

Fix race condition by reading prevNc inside lock.  Utility methods
correctly calculate min/max link bandwidth values.

Test: bit FrameworksNetTests:android.net.,com.android.server.net.,com.android.server.connectivity.,com.android.server.ConnectivityServiceTest
Bug: 68397798, 16207332
Change-Id: I3e1a6544c902bf3a79356b72d3616af1fd2b0f49