Remove NetworkStack test dependency on testutils

frameworks-base-testutils is close to unused in the tests, and has the
side-effect of adding some unrelated tests to be run in
NetworkStackTests like TestLooperTest. TestLooperTest is also failing in
some cases.

Bug: 127908503
Test: atest NetworkStackTests
Change-Id: Iebc174ed1362e8c341d765129a7fc51f8ed1999d