Various addon manager related fixes

 - The first commit makes sure all required arguments are passed to the corresponding `InstallListener`s as described on [MDN][1].
 - The second commit adds a comment to the `RemoteScript#install` call describing that this call also replaces the cached `ScriptAddon` object, thus explaining why the cached object can be used below.
 - The third commit properly sets `this._remoteScript` in `ScriptInstall#install`, so that `ScriptInstall#cancel` can actually call `this._remoteScript.cleanup()` when called.

[1]: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Add-on_Manager/InstallListener