Add new netd event callback for changes to private DNS validation state.

Test: None.
Bug: 71828272
Merged-In: Ib6824606b95deb23da2edd7aa4b96a0de66b12d1
Merged-In: Id88a4e846ae50536ac7b1541cf10c1fad1b1eeca
Change-Id: Ibf2683b9ff7c6cb69d7f4ad12b8bdff7cf7008f0
(cherry picked from commit 99c5029b91f17c46acd219a317b48b3ac0fe9a90)
3 files changed