Changed behaviour of the wallpaper picker.

When opening the picker, the currently chosen wallpaper is
shown in the background.
In this change also the nullpointer was fixed when deleting
the currently selected wallpaper and setting it, because we
revert to the old wallpaper in that case.
When going into the same live wallpaper than the currently
selected we are going back to home if the user sets it or
goes back now, in order for the user not to loose state.
The real fix for this will be done later when the following
bug will be fixed: b/13241760

Bug: 12063773
Bug: 13219612
Change-Id: I7c6abb25755eca99c3255278e0884d1ff4749b55