Adds an event handler that runs when the mobile picker element's value is changed.
A mobile picker element receives a change event when a mobile app user changes the value
in the mobile picker element.
A change event is not triggered when you change the mobile picker element's value using
the element's value property.
Because onChange() is fired before validations are performed, do not check
any of the validation properties in the onChange() event handler. For example,
do not code an if statement that checks the valid property in the onChange()
because the property at this point contains values prior to the onChange().
handler(event: Event): void The name of the function or
the function expression to run when the mobile picker element's value changes.