FieldLayout: Only use <label> for InputWidgets

InputWidget is the only widget guaranteed to contain exactly one form
element inside. Always using labels leads to things like nesting
<label/> elements or applying one label to multiple inputs.

Change-Id: I3a8c9d247426a04d663e00301cbad26d393b99a5
Merge "FieldLayout: Only use <label> for InputWidgets"