Make some gesture related slices public.

Only PreferenceControllers with isPublicSlice() set to true are exposed
to other apps. Others will be Settings only.

Bug: 141088937
Test: robotests
Change-Id: I0b54e752c5982f13193590db47dc3a84dde5311d

tests/robotests/src/com/android/settings/gestures/DoubleTwistPreferenceControllerTest.java

@@ -170,4 +170,9 @@ public class DoubleTwistPreferenceControllerTest {
         new DoubleTwistPreferenceController(mContext, "bad_key");
         assertThat(controller.isSliceable()).isFalse();
     }
+
+    @Test
+    public void isPublicSlice_returnTrue() {
+        assertThat(mController.isPublicSlice()).isTrue();
+    }
 }