fix by changing from 2 ways binding to 1 way binding of checked prope…

[Harry-Ph]

Fixes by changing from 2 ways binding to 1 way binding of checked property on radio-button, because after trigger _onChange on radio-button, the checked prop is updated and then it will trigger checked-changed event listener on radio-group and update its value on radio-group, and change event on radio-group is dispatched that includes updated value.
In the keyboard selection case, changed the order of checks the checkbox, causing the change event and update value of radio-group.

Fixes #125

[claassistantio]

All committers have signed the CLA.

[web-padawan]

Please update the tests to use async callbacks.

[web-padawan]

The PR does not fix the issue with change event on keyboard selection.

See the following GIF:

To reproduce it, please update the demo:

vaadin-radio-button/demo/radio-button-basic-demos.html

Lines 81 to 89 in e2c6697

	<div id="output">Selected value:</div>
	<script>
	window.addDemoReadyListener('#value-change-event', function(document) {
	const radioButtonGroup = document.querySelector('vaadin-radio-group');
	const output = document.querySelector('#output');
	radioButtonGroup.addEventListener('value-changed', function(event) {
	output.textContent = radioButtonGroup.value;
	});
	});

to use change event instead of value-changed. Then select and press arrow keys (up / down).

[web-padawan]

Some comments related to code style, please fix.