Bug 830644 - [wallpaper] Support WVGA and other non-HiDPI screen dimensions and resolutions

* Added @2x wallpapers
* px to rem conversions
* fixed header implementation

In order to test this PR you need to:
 * **HIDPI Device:** HIDPI=1 make install-gaia
 * **non-HIDPI Device:** make install-gaia
 * **B2G desktop:** run it with --screen=480x800 flag

This PR is a continuation of https://github.com/mozilla-b2g/gaia/pull/8910 and have the fixes addressed.
@davidflanagan  we can move the conversation here.