Re-hide onPreCheck and unhide NET_CAPABILITY_VALIDATED.

The API review comments in http://b/21343774 point out that the
suggested use case for onPreCheck (captive portal login apps) is
not a good use case as it requires that the app always be
running.

Also, unhide NET_CAPABILITY_VALIDATED, which is useful to apps
that want to detect captive portals and network connectivity
failures.

Bug: 21343774
Change-Id: Iad7c839bcc136b0fa9581dccc5fd97a28efed4ab