Improve handling of invalid private DNS settings

When private DNS mode is strict, there should always be a private DNS
specifier with the hostname.

Instead of restoring an invalid strict mode setting when set,
have tests reset private DNS mode to opportunistic and fail.

Bug: 190465704
Test: atest CtsNetTestCases
Change-Id: I45adc527267aa86d52e824f426699c5a7e874f63
1 file changed