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/DoubleTapScreenPreferenceControllerTest.java

@@ -178,5 +178,20 @@ public class DoubleTapScreenPreferenceControllerTest {
         final int availabilityStatus = mController.getAvailabilityStatus();
 
         assertThat(availabilityStatus).isEqualTo(AVAILABLE);
+
+    }
+
+    @Test
+    public void isSliceableCorrectKey_returnsTrue() {
+        final DoubleTapScreenPreferenceController controller =
+                new DoubleTapScreenPreferenceController(mContext,"gesture_double_tap_screen");
+        assertThat(controller.isSliceable()).isTrue();
+    }
+
+    @Test
+    public void isSliceableIncorrectKey_returnsFalse() {
+        final DoubleTapScreenPreferenceController controller =
+                new DoubleTapScreenPreferenceController(mContext, "bad_key");
+        assertThat(controller.isSliceable()).isFalse();
     }
 }