Hide required underlying caps APIs

VCN underlying network capabilities should be transport-dependent in
order to allow using anything other than the INTERNET capability for
VCN types that support wifi offload. Specifically, if underlying
network capabilities are not transport-dependent, and Wifi only ever
supports the INTERNET capability, the VCN is unable to utilize wifi
offload together with requiring NET_CAPABILITY_IMS or
NET_CAPABILITY_CBS, since the IMS or CBS capability would be required
for both cellular and wifi underlying networks.

Until such time as a per-transport capability set is allowed, hide
the exposedCapability pieces, and document that all underlying networks
MUST have INTERNET capability in order to be used.

Bug: 182219992
Test: atest FrameworksVcnTests
Test: atest CtsVcnTestCases
Change-Id: I50d7f1be42e0e001f1413a3d5fe8aa4b7afec223