Remove Slice getter from BasePreferenceController

After working through a prototype here: (ag/3324435)
it is clear that we don't need the controller to
provider the slice. We will build an index that
will contain sufficent UI information, and a reference
to the controller.

At Slice Bind time, we can get the curret value from the
controller, and the UI information from the Index.

Bug: 67996923
Test: robotests
Change-Id: Id43a51bcd73051bc719cd5829907583e0edf23b2