[Maps] Do not use MAP_SAVED_OBJECT_TYPE constant when defining URL paths

[nreese]

The constant MAP_SAVED_OBJECT_TYPE is being used for multiple meanings

1. saved object type
2. defining the path of a route at https://github.com/elastic/kibana/blob/master/x-pack/plugins/maps/public/maps_vis_type_alias.js#L31 and https://github.com/elastic/kibana/blob/master/x-pack/plugins/maps/common/constants.ts#L35

Even though both uses use the same string, they are not the same concepts and MAP_SAVED_OBJECT_TYPE should be split into two constants to clearly define the concepts for each value.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)