threadnetwork: Avoid execute command if service does not exist

We might run VTS when the thread_network service is not available. This
patch prevents shell commands from being executed if the service is
absent.

Bug: 374009923
Bug: 355317617
Bug: 376750874
Test: atest VtsHalThreadNetworkTargetTest
Change-Id: I94dd7e2e7585e061f53291c47e46841e5e670b6c
(cherry picked from commit ae9ea3c8966eace86a12b1da53295957f411aa4d)
1 file changed