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/gestures/DoubleTwistPreferenceControllerTest.java

@@ -155,4 +155,18 @@ public class DoubleTwistPreferenceControllerTest {
 
         assertThat(mController.isChecked()).isFalse();
     }
+
+    @Test
+    public void isSliceableCorrectKey_returnsTrue() {
+        final DoubleTwistPreferenceController controller =
+        new DoubleTwistPreferenceController(mContext,"gesture_double_twist");
+        assertThat(controller.isSliceable()).isTrue();
+    }
+
+    @Test
+    public void isSliceableIncorrectKey_returnsFalse() {
+        final DoubleTwistPreferenceController controller =
+        new DoubleTwistPreferenceController(mContext, "bad_key");
+        assertThat(controller.isSliceable()).isFalse();
+    }
 }