Bug 1040070 - [Verical Homescreen] Use smaller images sizes for low capability devices r=crdlc

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.
Merge pull request #21834 from KevinGrandon/bug_1029902_low_memory_features

Bug 1029902 - [Vertical Homescreen] Implement device capability configuration