Bug 975293 - Make sure we have _containerElement defined in the context ...

...of _onTransitionEnd

1. Make sure we have `_containerElement` defined in the context of `_onTransitionEnd()`
2. Coerce `cardIndex` to number for l10n usage (otherwise we'll get string like **SIM 11** instead of **SIM 2**)

https://bugzilla.mozilla.org/show_bug.cgi?id=975293