Clarify docs, sanity check roaming state.

Update public docs to hide the fact that NetworkCapabilities is only
used inside NetworkRequest as an implementation detail.

Take up less room on the wire when passing NetworkCapabilities around
via NetworkRequest.

Sanity check that the roaming state between NetworkInfo and
NetworkCapabilities is in agreement.

Test: bit FrameworksNetTests:android.net.,com.android.server.net.,com.android.server.connectivity.,com.android.server.ConnectivityServiceTest
Bug: 67040695
Change-Id: I982b4c3c41a140934bbad3b8ca8f12dc3814e86c