Separate the integration root test.

To separate the IpClientRootTest out of the current integration test
suite, make it possible to run NetworkStackRootTest on OEMs' devices
without required network_stack certficate and permissions. And add
NetworkStackRootTest to MTS with mts-network tag.

Bug: 203175155
Test: atest NetworkStackIntegrationTests
      atest NetworkStackNextIntegrationTests
      atest NetworkStackRootTests
      atest NetworkStackCoverageTests
Change-Id: Ic042621938fb7f677e6fb288e2d30de586269a28
9 files changed