setUnderlyingNetworks() is mainly for the NetworkAgents who hold
the NETWORK_FACTORY to set its underlying networks.
And the underlying networks are only visible and useful for the
caller of getNetworkCapabilities() or the receiver of
onCapabilitiesChanged() who hold one of NETWORK_FACTORY,
NETWORK_SETTINGS and MAINLINE_NETWORK_STACK permissions.
Otherwise, the underlying networks field will be cleard before
sending.
Bug: 205738644
Test: atest CtsNetTestCases:ConnectivityManagerTest
atest CtsHostsideNetworkTests:HostsideVpnTests
atest FrameworksNetTests
Change-Id: Ife7630d9676a31ee5ab977cb1b87aec3b6fd7080
69e1aa9117