Fix Dispatchers in DatePicker and TimePicker.

Call AfterDateSet/AfterTimeSet (which dispatches an App Inventor
event) via the Android handler mechanism to ensure it is called after
the Picker’s event is finished. This permits the App Inventor event to
display another Picker based on the current instance.

Change-Id: Ic1674b778bdd1c8e0f7606752136d5e570f9b2ae