Allow custom wallpaper and boot animation.

Add new "ro.config.wallpaper" and "ro.config.wallpaper_component"
properties which may be defined outside of the bundled framework
resources.  Falls back to bundled resources when properties are
undefined.

Also look for boot animation under OEM partition.

Bug: 13340779
Change-Id: Ibdc9935dbdaae3319bf63b40573de0503d82ae67