Add a helper function to get service runtime

The bufferhub binder service would be used a lot after migrated to
binder, so have a helper function and including all the checks inside it
should help a lot.

Also rewrite the test to use and check this function as well.

Test: "atest buffer_hub_binder_service-test" passed.
Bug: 112338294
Change-Id: Ifa66f25cc284b2c03586b8dc75e05801bf393fa6