DataViews density option: preserve selection across layout selection

[andrewserong]

Follow-up to:

• DataViews Grid and Picker Grid: Add density option for gap between items #75887

Review feedback on the above PR noted that now that each layout type has a density option, selecting a density option should be preserved as a user switches between layout types. This is the control:

From the comments (#75887 (comment)):

The way I envision this is that we just make density part of the ViewBase type. The edge case we need to check is what happens when there's a stored view (e.g., user has saved some changes) and we introduce this change.

I think another thing to double-check is:

• If an instance of the DataViews defines its own value for density and a user has made no selection, the value defined by that DataViews instance should take priority
• For that particular DataViews instance (e.g. media library modal, etc), if a user has made a selection (or preserved preference), then that preference takes precedence over the value provided by that view