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: I8a60b6f02dd0f42268b59690556c16335d34e220