Add Gestures settings to search index.

Add GestureSettings class to the resource map of
SearchIndeSearchIndexableResources so that the preferences in the
gestures settings will be indexed and showed up as search result.

Bug: 30169776
Change-Id: I51be9e82dc8bce93e2abd13b48ceb3cd051cbcf4

src/com/android/settings/gestures/GestureSettings.java

@@ -230,6 +230,9 @@ public class GestureSettings extends SettingsPreferenceFragment implements
                 if (!isSystemUINavigationAvailable(context)) {
                     result.add(PREF_KEY_SWIPE_DOWN_FINGERPRINT);
                 }
+                if (!isDoubleTwistAvailable(context)) {
+                    result.add(PREF_KEY_DOUBLE_TWIST);
+                }
                 return result;
             }
         };