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: I42f0d9b17b3058f6e10fd2e651278b290f26667f