[Maps] Mark instance variables as readonly

[thomasneirynck]

All instance-variables in Maps under layers/**/*.ts should be marked as readonly.

The current architecture of Maps expects it:

• all state needs to be represented in the redux store
• there's only private instance-state, since all public access is mediated through methods on the interface.

This is now implicitly enforced through code reviews.

By using the readonly keyword, we can make this explicit and enforce it at compile-time.

This is for typescript code only.

[elasticmachine]

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