Add NDK API for getprocdns, setprocdns

The API allows callers to control the default network that is used for
DNS hostname resolution.

Test: atest CtsNetTestCases
      Tests in change I00143cafcd3eb1d71e8d5c7ea9c839a99dc6f4ce
Bug: 171540887
Change-Id: I70d8113ae49cd1e0910a6dbc5929b6a157c44f42