Code review comment for ~knobby/layer-snap:mwilson/is-installed

The idea is fine. I need the get_installed_state_name and get_disabled_state_name function names changed to match recommended nomenclature before I can land it (get_installed_flag and get_disabled_flag rather than _state_name)

My use cases just hard code the documented flag names, but I can see that the new methods will be useful when the set of snaps is configurable.