Issue 1756 - preparation to open shared decks in browser

This is some preparation to get shared decks downloadable by browser.
There's an issue in newer versions of Android where the Android DownloadManager has [apparently stopped providing absolute links to its downloads](http://stackoverflow.com/questions/14364091/retrieve-file-path-from-caught-downloadmanager-intent). Instead it passes a link like "content://downloads/all_downloads/166". So we currently just get a message saying "can't open file".

So until we can resolve this issue, I'll keep the old method enabled, but this is an important step towards eventually getting there.