Bug 1029902 - [Vertical Homescreen] Implement device capability configuration

The following changes are made for low capability devices:

* Icons default to 4 per row.
* Icon resolution is reduced.
* Icon rendering does not account for devicePixelRatio.

https://bugzilla.mozilla.org/show_bug.cgi?id=1029902
I think that we have to check that defaultCols is not undefined before putting it as a preference