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/core/BasePreferenceController.java

@@ -234,6 +234,23 @@ public abstract class BasePreferenceController extends AbstractPreferenceControl
         return null;
     }
 
+    /**
+     * Determines if the controller should be used as a Slice.
+     * <p>
+     *     Important criteria for a Slice are:
+     *     - Must be secure
+     *     - Must not be a privacy leak
+     *     - Must be understandable as a stand-alone Setting.
+     * <p>
+     *     This does not guarantee the setting is available. {@link #isAvailable()} should sill be
+     *     called.
+     *
+     * @return {@code true} if the controller should be used externally as a Slice.
+     */
+    public boolean isSliceable() {
+        return false;
+    }
+
     /**
      * Updates non-indexable keys for search provider.
      *