Add "not congested" network capability.

Can be used by a network to indicate when it's congested, meaning that
apps should defer network traffic that could be done at a later time.

Test: bit FrameworksNetTests:android.net.,com.android.server.net.,com.android.server.connectivity.VpnTest,com.android.server.ConnectivityServiceTest
Bug: 64133169
Change-Id: Icfaff89c6ba15ccb6108ac95a52b3744c3c8a4b0