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.
     3. Add 1.5x graphics (not complete, some assets are still missing)
Merge pull request #10947 from RudyLu/homescreen/v1.1.0hd

Bug 881149 - hdpi/xhdpi resolution assets and layout for Homescreen app. 
  r=crdlc. a=hd+.