Bug 903915 - [Flatfish][Homescreen] add support for large device

*   To test this patch on nightly:
    1.  ````$ make DEBUG=1 GAIA_DEV_PIXELS_PER_PX=2```` (*)
    2.  to set responsive design window to 1280 x 800 px, enter ```about:config``` and set ```devtools.responsiveUI.presents``` to ```{“key”:”1280x800”, “width”:1280, “height”:800}```

(*) add `GAIA_DEV_PIXELS_PER_PX=2` option is a temp solution for large device, will followed by `adaptive image support` patch to allow show different size image in same density