For implementing desktop UIs, higher density layouts are necessary. Layouts meant for touch devices are much larger/looser in their layout than desktop layouts want to be, by virtue of having a much smaller pointer size (mouse pointers are more accurate than fingers).

We need to develop an API for setting vertical and horizontal density of our Material and Cupertino controls, in a way that is backward compatible and easy to set up. It should default to the existing layout density on mobile, and to a much tighter density on desktop platforms.

Implementation of the automatic setting of this mode is predicated on the existence of a way to determine which platform is being targeted, so it depends up on #31366