Test that IpClient does not leak fds on shutdown.

This CL adds a test that repeatedly shuts down and restarts
IpClient and checks that the number of open FDs was the same as
it was before.

Bug: 161330494
Test: reverting aosp/1364277 makes test fail
Test: atest NetworkStackNextIntegrationTests:IpClientIntegrationTest
Test: atest --rerun-until-failure 100 NetworkStackNextIntegrationTests:IpClientIntegrationTest#testNoFdLeaks
Change-Id: If6f9204e221df200dc762508ba14ff7150c8835a