Move getTestableNetworks method to CtsNetUtils

This is a no-op change to move the getTestableNetworks() to
CtsNetUtils for the follow up change which will also need the
same method to reduce code duplication.

This change also updates the nullability of the private dns
server so that it can also verify non-strict mode.

Test: atest CtsNetTestCases
Change-Id: I67c94cd5aefeb9cf1ecd4126178c2410f3f08b1b
2 files changed