Bug 893966 - some 1.5x assets are still missing for Homescreen app
 1. Add 1.5x assets.
 2. Modify the background size for app delete icon.
Merge pull request #11044 from RudyLu/homescreen/Bug893966-add_1.5x_assets

Bug 893966 - some 1.5x assets are still missing for Homescreen app. r=rexboy7.