Expose Intent#ACTION_CLEAR_DNS_CACHE

DnsManager will broadcast ACTION_CLEAR_DNS_CACHE intent to clear
dns cache when network have changed. But DnsManager is in the
Connectivity mainline module which can not use non-formal API.
Thus, expose Intent#ACTION_CLEAR_DNS_CACHE as module lib API for
mainline module.

Bug: 182370679
Test: m update-api
Change-Id: I30b1b146dcae82926be7bdb8b343635a01eabefe