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

tests/robotests/src/com/android/settings/core/BasePreferenceControllerTest.java

@@ -117,4 +117,4 @@ public class BasePreferenceControllerTest {
 
         assertThat(mPreferenceController.isSupported()).isTrue();
     }
-}
+}