Add a feature flag to control if accepting the IPv6 link-local DNS.

The default feature flag is true which doesn't change the current
behavior, just providing a function method which allows us to push
a different flag value from g3 and does not accpet the IPv6 link-local
dns, then we can do A/B testing on beta devices to see if the fix does
work as expected.

Bug: 163492391
Test: atest NetworkStackIntegrationTests
Test: atest NetworkStackRootTests
(cherry picked from https://android-review.googlesource.com/q/commit:eedf05b688aa3d910089b2305312dd488e7150d8)
Merged-In: I405d023711ad0c4c71541675eeda8cd2582e1bdc
Change-Id: I405d023711ad0c4c71541675eeda8cd2582e1bdc
3 files changed