[Lens][Embeddable refactor] Follow ups

[dej611]

This is a set of follow ups after the Lens Embeddable refactor gets merged:

• [ ] rename getState into the embeddable factory into getEmbeddableState
• [ ] move the visualizationContext logic into the internalApi
• [ ] revisit the LensApi types and reduce its surface as much as possible
• [ ] move the Lens inline editor from trigger-based into a separate component (like the Lens embeddable one)
• [ ] Write more embeddable tests (borrow from the old one where possible)

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)