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

[gkalpak]

ngAria's ngClick (among other things) tries to emulate the behavior of native buttons for non-button elements (e.g. <div>, <li> etc) used as button. Using native buttons is strongly recommended, but we should still try to make things accessible, even if the developer chooses to not follow the best practice 😃

Currently, we detect "clicks" by listening to keypress events. Switching to keydown instead, would make the behavior more consistent with how the browser fires click events on native buttons.

Also, discussed in https://github.com/angular/angular.js/pull/13996/files#r52934300.