Support Slice type in SliceData

Add a getter for Slice type for BasePreferenceController.
This facilitates the generation of all possible Slices for Settings
search and discovery for AGSA.

Bug: 62807132
Test: robotests
Change-Id: I2cba1fc72c95769ecc26c76d1f2faa49a285d077

tests/robotests/src/com/android/settings/slices/FakePreferenceController.java

@@ -30,4 +30,9 @@ public class FakePreferenceController extends BasePreferenceController {
     public int getAvailabilityStatus() {
         return AVAILABLE;
     }
+
+    @Override
+    public int getSliceType() {
+        return SliceData.SliceType.SLIDER;
+    }
 }