Fix default value problem of DnsResolver Experiments#getFlag()

The default value of Experiments::getFlag() doesn't actually work
because Experiments::updateInternal() wil also initial the property as
0. Use INT_MIN as the default value to represent the flag can't be
found.

Bug: 155042187
Test: atest
Change-Id: Icc863b9a9d7b4cefca4dce23933c705a51299095