How to preview all UI elements/widgets and dialogs I need to view all the different UI elements and dialogs on a device. This would be helpful while developing new platform level themes.

Is there an existing app that will do this? Are there other ways to view all the UI elements (e.g. android.widget.Button, android.widget.CheckBox, etc).
You should take a look at the Android API Samples. 

The APIDemos application is exactly what you are looking for:
http://developer.android.com/resources/samples/ApiDemos/index.html