autotest:  Reference count the start/stop servod calls.

If start servod is called more than one time the actual container
is not started so it is a noop.

However if stop is called and the container is running we shutdown
the process.

This can leave a servo_host object that thing start has been called
but the container is not running.

BUG=b:201425201
TEST=local build

Change-Id: I78faa844d4fdc590a66b3caf59381b818b70437a
2 files changed