Finish moving NetworkStats tests to tests/net

Follow-up of commit c86013be936f36c61fa0cdfad95141f6de3c6ef7.

Also add @SmallTest annotation to classes moved to tests/net.

Test: no functional change
Bug: 32561414
Change-Id: I0a0e8865d37ba7bae06ce352d3fc385989adc300
6 files changed