demos: Omit value for disabled attribute, use attr() instead of prop()

Follows-up ebfa365517. See also I8ad51219.

Also optimised class toggling logic by re-using the query result
instead of querying the body twice.

Change-Id: I52d69cdaa65bdf5ac0e4a76f9c88c3c41959c84d
Merge "demos: Omit value for disabled attribute, use attr() instead of prop()"