Allow method to reset/start network proxy log after creation of TransparentProxyService

After https://github.com/google/oss-rebuild/pull/113, the user is able to create a TransparentProxyService struct that does not do in memory logging. We should add a function to start logging or reset the log if it already exists. To do this we will also need to kill existing goroutine inside of CaptureActivityLog if running.