Android performance improvements.

Only refresh visible wheels.
Fixes #98

android/src/main/java/com/henninghall/date_picker/PickerView.java

@@ -176,6 +176,7 @@ public class PickerView extends RelativeLayout {
         applyOnAllWheels(new UpdateVisibility());
         wheelOrderUpdater.update(locale, mode);
         emptyWheelUpdater.update(mode);
+        requireDisplayValueUpdate = true;
     }
 
     public Collection<Wheel> getVisibleWheels() {

android/src/main/java/com/henninghall/date_picker/wheels/Wheel.java

@@ -85,6 +85,7 @@ public abstract class Wheel {
     }
 
     public void refresh() {
+        if (!this.visible()) return;
         clearValues();
         init();
     }