Add 'choice' field to dynamic_ui list of fields

Markup:
 - name: choiceSample
   label: My Choice
   type: choice
   choices:
     - [value1, text1]
     - [value2, text2]
   initial: value2

Change-Id: I4d8c76c831140c3c7fbb36e6735d1258b096d4cb
Merge "Add 'choice' field to dynamic_ui list of fields"