Stop netd before running tests on Buildbot devices.

When netd is running, some libcore and JDWP tests fail with this
exception:

  android.system.ErrnoException: connect failed: EBADMSG (Not a data message)

Turn it off to make these tests pass.

Test: art/tools/run-libcore-tests.sh --mode=device
Test: art/tools/run-jdwp-tests.sh --mode=device
Bug: 74725685
Change-Id: If54723822ccdaf18fd684a1b61f1d1d26acfee3d