Refactor TestNetworkCallback out of ConnectivityServiceTest.

TestableNetworkCallback will become a standard tool. This patch
and its companion in packages/modules/NetworkStack mostly move
it out of ConnectivityServiceTest, but also applies a few
cleanups like reordering arguments for standard order, renaming
...Like to ...That to match Mockito namings, and introduces
stonger static typing on some callbacks, which allows to remove
some unchecked casts.

Test: atest FrameworksNetTests NetworkStackTests
Change-Id: I808a95ac33ec0c81a8829cad36a6917524297574