Add an API to flush cache on designated network

Flush entire cache on a specified network. The API acquires lock before
manipulate cache. It's thread-safe API.

Bug: 139646101
Test: atest resolv_cache_unit_test.cpp#FlushCache
Test: atest resolv_integration_test.cpp#FlushNetworkCache

Change-Id: I4ea34a256013468ceac21ce5067d6a493d8631f8