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
Change-Id: I68665aaf74b7d59182cc6f9586b80b55b0dfe427