[Lens] Use pie instead of donut as default and stop switching based on number of rings

[wylieconlon]

There are two changes we can make to match user expectations more closely in Lens:

• For transitions from other chart types, use a pie suggestion instead of a donut suggestion

• For transitions within the partition charts, stop rewriting the shape based on the number of rings. There are two cases where this should be tested:

  • Open an empty pie chart and add a suggestion. It should keep you on pie instead of switching to donut.
  • Open a single-ring donut chart and add a second ring. It should keep you on donut instead of switching to pie.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)