Checks that icons follow the recommended visual style

Notification icons and Action Bar icons should only white and shades of gray. See the Android Design Guide for more details. Note that the way Lint decides whether an icon is an action bar icon or a notification icon is based on the filename prefix: ic_menu_ for action bar icons, ic_stat_ for notification icons etc. These correspond to the naming conventions documented in http://developer.android.com/guide/practices/ui_guidelines/icon_design.html