bug 922006 - [Dialer] Refactor onload event listener to make dialer.js testable

* Move out all `onload` listener from `dialer` and `keypad.js` to individual `index.js`
* Move out all `onload` listener from `calls_handler.js ` to individual `oncall.js`

Some `onload` listener are merged into `init()`, or just taken away if they are already called after `onload`.