Improve hidden field validation

Hidden fields are supposed to be ignored during validation; more
completely ensure this by checking in HTMLForm::trySubmit before calling
validate.

Also, more properly handle HTMLCheckFields in the isHidden check by
casting their booleans to strings.

Bug: 68132
Change-Id: I84f8239c299727b773015643048eed4684417733
Merge "Improve hidden field validation"