Add NetworkStackTests to mts

The test_suites tag is required so that NetworkStackTests is built as
part of the mts package. NetworkStackTests is run for coverage in mts,
but is not part of the main mts test suite to run on release devices.

Test: m mts && mts-tradefed run mts-network-coverage
Change-Id: I11336e86156d62b135ca19445caa520c71699346