[Maps] Consider making grid rectangles the default for grid agg layer

[bcamper]

We've have a few cases where people are confused by our default Grid Aggregation layer behavior, which is to show the grid agg as points ("Show as: points" in the UI). This option first does a geo-tile agg, then does a centroid agg for all the points within each each grid cell. This can be misleading because the points can look like they are meant to represent actual documents (they look very similar to the Documents layer, and traditionally a circle symbol will more commonly represent a single entity).

The grid rectangle and heatmap options are more likely to make the aggregation/document distinction clear (heatmaps are problematic for other reasons though! :).

I propose we change the default to "Show as: grid rectangles", which looks like this (individual Documents layer shown on top for context):

[elasticmachine]

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