Trigger events from netd upon private dns validation successes

and failures.

The new events are handled in aosp/591294.

Test: Have observed validation events being sent by netd on a Pixel.
Bug: 71828272
Merged-In: Ie8c9c3df14998523a4b46bf0177c327569c9967a
Merged-In: I6f5cf7a235633f94de58fcce768526d5a344d253
Change-Id: Iba9fb2bcb9b515be71f4b2d71f3a9208bb329d05
(cherry picked from commit 6250dbdd)