Remove SkipMainlinePresubmit annotation from testIsPrivateDnsBroken.

testIsPrivateDnsBroken isn't flaky any more, remove the
SkipMainlinePresubmit annotation.

Bug: 289879947
Test: atest ConnectivityManagerTest#testIsPrivateDnsBroken --iterations 100
Change-Id: I402153fa09144a96a500c96ecf7c89f41702bd0d
1 file changed