Prepare exposing Network{Request,Caps}.forbiddenCapabilities

This is useful going forward in particular because there will
be a default forbidden capability for LOCAL_NETWORKS. This
means it will be useful to be able to remove it.

It is also generally useful and we have been wanting to open
this API to the public, so this is a good opportunity to do so.

Test: new tests for NetworkRequest
      NetworkCapabilitiesTest already has tests because it
      already was @SystemApi
Change-Id: Ibb8d33b799f2d274326fd9cd0b05a2c33a18032a