[VCN06] Support request background network

This will be mainly used by VCN management service which will
need to hold the networks but preserve the backgrounded-ness
of the networks.

Test: android.net.ConnectivityManagerTest#testRequestType
Test: android.net.cts.ConnectivityManagerTest#testRequestBackgroundNetwork /
      --rerun-until-failure 100
Test: ConnectivityServiceTest#testBackgroundNetworks
Test: m -j doc-comment-check-docs
Bug: 175662146
Change-Id: If9aaa87b7e71c2b695ac7b08858850e975b28bb6