Add NDK API for getprocnetwork

The API is the getter couterpart for setprocnetwork.
Use it in NetworkUtils so that the NDK API can be the source of truth
for the process network.

Bug: 171540887
Test: atest CtsNetTestCases
      Tests in change I311b58585033c2ca50ce5477ea9cd94b6f127507
Change-Id: Ie8f68cf1fa57deddb63324c1abf3d6fd5b0ef500