Add support for registering test networks with administrators.

CTS testing for ConnectivityDiagnosticsManager requires registering
TestNetworks with the administrator UIDs set. This sets up the CTS
process to be an administrator over the test network, which allows it to
receive ConnectivityDiagnosticsCallback callbacks.

Bug: 148032944
Test: atest android.net.cts.ConnectivityDiagnosticsManagerTest
Test: atest IpSecManagerTunnelTest
Change-Id: I7a443c368cf1ac470fbe40883ca24bc3143153b0