Fix leak of NetworkAgentWrapper HandlerThread

ConnectivityServiceTest creates hundreds of NetworkAgentWrapper, without
stopping their HandlerThread. This leaks a lot of threads while tests
are running.

Ensure the threads are stopped in ConnectivityServiceTest, and
integration tests that also use NetworkAgentWrapper.

Test: atest, watch adb shell ps -T [test package pid]
Bug: 310659032
Change-Id: I34f367b2efe3d05e1db5eab4c9fd93ee79c1b683