Trigger NetworkCallback events when private DNS usage has changed.

Relies on events sent from netd in aosp/578162.

Test: Added tests to ConnectivityServiceTest. Added a new test
class DnsManagerTest. Built a simple app that appears to
receive onLinkProperties events correctly upon manual changes
to the private DNS settings on a Pixel.
Bug: 71828272

Merged-In: I1e6c54ba016f6a165a302bd135a29d9332aaa235
Merged-In: I7705412803fb9aa707a18ae5a1c50292e084d851
Change-Id: I3223c1285a73d5d531c5051ce70007857caa57e3
(cherry picked from commit 7301aa41)