When private DNS setting was set to opportunistic (mode) and null
(hostname), CtsNetUtils would not restore it.
Make sure that private DNS settings are restored after every test. Also
fail if restore is called without having saved any setting beforehand.
Bug: 190465704
Test: atest CtsNetTestCases
Original-Change: https://android-review.googlesource.com/1729162
Merged-In: Ic5d8d8b729469e0eef89a0b53f166e604264c1ee
Change-Id: Ic5d8d8b729469e0eef89a0b53f166e604264c1ee
7fd2b373ff