Add transparency settings for lockscreen (2/2)

This will enable the user to set the transparency level
for the lockscreen.

Change-Id: I624b2e136432bb0effceac9f7d41c78c65d22125
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>
Revert "Use RGB565 surface for wallpaper"

This reverts commit 69a5e4808ad927105e4e099eb7bfc631a0d467fd.
Revert "Prefer 16bpp formats for opaque bitmaps"

This reverts commit e42b690d6f2b24860e56cfc8d2210f53866e10e1.