Move ExternalPrivateDnsStatus away from libnetd_resolv's public headers

This is a first pass at sanitizing the public headers of libnetd_resolv by moving out things that are only used internally.

The main motivation is reducing noise in ABI dumps generated for the ABI checker. It's also a nice opportunity to cut off the C wrappers that we no longer need.

Bug: 133117690
Test: cd system/netd && atest
Merged-In: I08689c263c4fd96363666a108ba08d768ad327df
Change-Id: Id0ab90fe0f962ebaf02e1cdd740ff7c04ef02eae
(cherry picked from commit 1a80be40)