Add VcnUnderlyingNetworkSpecifier

This change adds a VcnUnderlyingNetworkSpecifier as hidden API, for
internal use only. The purpose of the VcnUnderlyingNetworkSpecifier is to
match the specifiers for ONLY underlying (Telephony|Wifi) networks,
ensuring that the VCN does NOT create a routing loop

Bug: 179439329
Test: atest FrameworksVcnTests
Test: atest android.net.vcn
Change-Id: I0e158e1889d4462c117c46d9038f58d5adcc7e30