fix(ngAria): bind to `keydown` instead of `keypress` in `ngClick`

Previously, `ngAria` would provide keyboard support for non-native buttons (via `ngClick`), by
binding the `ngClick` handler to the `keypress` event. In an attempt to better emulate the behavior
of native buttons, `ngAria` will now bind to the `keydown` event (instead of `keypress`).

The configuration flag for this feature has been renamed from `bindKeypress` to `bindKeydown`, to
closer describe the underlying behavior.

Fixes #14063
Closes #14065

BREAKING CHANGE:

If you were explicitly setting the value of the `bindKeypress` flag, you need to change your code to
use `bindKeydown` instead.

Before: `$ariaProvider.config({bindKeypress: xyz})`
After: `$ariaProvider.config({bindKeydown: xyz})`

**Note:**
If the element already has any of the `ngKeydown`/`ngKeyup`/`ngKeypress` directives, `ngAria` will
_not_ bind to the `keydown` event, since it assumes that the developer has already taken care of
keyboard interaction for that element.

Although it is not expected to affect many applications, it might be desirable to keep the previous
behavior of binding to the `keypress` event instead of the `keydown`. In that case, you need to
manually use the `ngKeypress` directive (in addition to `ngClick`).

Before:

```html
<div ng-click="onClick()">
  I respond to `click` and `keypress` (not `keydown`)
</div>
```

After:

```html
<div ng-click="onClick()" ng-keypress="onClick()">
  I respond to `click` and `keypress` (not `keydown`)
</div>
<!-- OR -->
<div ng-click="onClick()">
  I respond to `click` and `keydown` (not `keypress`)
</div>
```

Finally, it is possible that this change affects your unit or end-to-end tests. If you are currently
expecting your custom buttons to automatically respond to the `keypress` event (due to `ngAria`),
you need to change the tests to trigger `keydown` events instead.