Race condition with download/update of pack

[lucasrowe]

The code that synchronizes packs when an amazon IAP response returns crashes if the user isn't on the Pack Purchase page.

To Reproduce:

1. Make sure an Amazon sync should occur. This can be done by purchasing something, deleting the app, and reinstalling the app.
2. Navigate very quickly past the Pack Purchase page, which will fire off the request to sync up IAPs.

Be sure to also test pack updates which take place outside of the Amazon response