Themes: Use first asset entry for lockscreen wallpaper

The current implementation assumes the wallpaper will be named
either wallpaper.png or wallpaper.jpg.  If this is not the case
the wallpaper is not applied.  This patch implements the lockscreen
wallpaper the same way that the regular wallpaper is done.

Change-Id: I9f3dd299c1e61158fa53c163f7e42e21e2f9b1dc
Merge remote-tracking branch 'cm/cm-11.0' into cm-11.0-sgt7