Clean up AssistGesturePreferenceController

The preference controller shouldn't be shared between the
"Assist Gesture" parent setting, and the child setting within

Fixes: 64318213

Test: manual test of settings && make -j RunSettingsRoboTests
Change-Id: I2d2437e2036881a08977924dc1386aa1fab67070

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

@@ -61,13 +61,6 @@ public class AssistGestureSettingsTest {
                 .isEqualTo(R.xml.assist_gesture_settings);
     }
 
-    @Test
-    public void testGetPreferenceControllers_shouldAllBeCreated() {
-        final List<AbstractPreferenceController> controllers =
-                mSettings.getPreferenceControllers(mContext);
-        assertThat(controllers.isEmpty()).isFalse();
-    }
-
     @Test
     public void testSearchIndexProvider_shouldIndexResource() {
         final List<SearchIndexableResource> indexRes =