Add isSliceable API to BasePrefController

Only support explicitly approved Settings Slices,
dictated by controllers which return true for the new
method isSliceable.

Updating the supported settings to a whitelist means that
the method to return all available slices must be updated,
and checking slicability when we index slices.

Test: robotests
Bug: 79779103
Change-Id: Ib2b9690cdd0036b5cc4a1cb846c52bce7c824ab9

src/com/android/settings/accessibility/AccessibilitySlicePreferenceController.java

@@ -90,6 +90,11 @@ public class AccessibilitySlicePreferenceController extends TogglePreferenceCont
         return getAccessibilityServiceInfo() == null ? UNSUPPORTED_ON_DEVICE : AVAILABLE;
     }
 
+    @Override
+    public boolean isSliceable() {
+        return true;
+    }
+
     private AccessibilityServiceInfo getAccessibilityServiceInfo() {
         final AccessibilityManager accessibilityManager = mContext.getSystemService(
                 AccessibilityManager.class);