Bug 881149 -  hdpi/xhdpi resolution layout for Homescreen app
     1. adjust to show 4 x 5 grid on WVGA.
     2. modify the scale ratio of app icon according to
        window.devicePixelRatio.
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     1. To address review comment, for reset the hidden of the 17th icon
     2. Add 1.5x graphics (not complete, some assets are still missing)
Merge pull request #10939 from RudyLu/homescreen/4x5grid_onWVGA_support

Bug 881149 -  hdpi/xhdpi resolution layout for Homescreen app. r=crdlc.