Expose constants of ConnectivityManager

The callers cannot call the hidden APIs after ConnectivityManager
became a part of mainline module, so expose them for callers.
Also change the value of ACTION_PROMPT_PARTIAL_CONNECTIVITY,
ACTION_PROMPT_LOST_VALIDATION and ACTION_PROMPT_UNVALIDATED because
of API lint errors.

Bug: 172183305
Test: Check private DNS settings is normal, and test NO_INTERNET
      notification can be shown normally.
Change-Id: I715c766ad8e5eb54f4dc67239c1dbca7239506fc