Rework multiple value power widgets (1/2)

The current batch of multi-value power widgets use a set of possible combinations
for cylcing, i.e. Auto/Dim/40%/100% for brightness. Instead, we can leverage
the MultiSelectListPreference to allow the user to select which values he or she
would like.

Change-Id: I1fb03a39ab75bd32fbf0a5de9e0802c393603e57