[Telemetry] Close network_controller once the browser tests are finished
[Telemetry] Close network_controller once the browser tests are finished

https://codereview.chromium.org/2250583004 makes telemetry always
start ts_proxy_server anytime a platform instance is created.
However, this means that at the end of each test that involves
query platform, we need to make sure that the network controller
is properly closed to clean up the ts_proxy_server.

BUG=catapult:#2584

Review-Url: https://codereview.chromium.org/2259273002

benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,benschmaus/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult,benschmaus/catapult,sahiljain/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,sahiljain/catapult,benschmaus/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult