Bug 1043121 - Move IMESwitcher in keyboard_manager to another script

The usual pattern in system app is

    this.imeSwitcher = new IMESwitcher(this);
    this.imeSwitcher.start();

and `IMESwitcher` will call `this.app.showAll()` when it needs to.

Or do the callback approach.

    this.imeSwitcher = new IMESwitcher(this);
    this.imeSwitcher.ontap = this.showAll.bind(this);
    this.imeSwitcher.start();