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

tests/robotests/src/com/android/settings/testutils/FakeUnavailablePreferenceController.java

@@ -18,4 +18,9 @@ public class FakeUnavailablePreferenceController extends BasePreferenceControlle
         return Settings.Global.getInt(mContext.getContentResolver(),
                 AVAILABILITY_KEY, 0);
     }
+
+    @Override
+    public boolean isSliceable() {
+        return true;
+    }
 }