Add help url for gestures settings.

Override getHelpResource() in GestureSettings and added corresponding
resource string for the help url.

Bug: 29792538
Change-Id: I2f2d93fc25b63266504bd13b954fefa5692e9ddf

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

@@ -109,6 +109,11 @@ public class GestureSettings extends SettingsPreferenceFragment implements
         return true;
     }
 
+    @Override
+    protected int getHelpResource() {
+        return R.string.help_url_gestures;
+    }
+
     @Override
     protected int getMetricsCategory() {
         return MetricsEvent.SETTINGS_GESTURES;