Fix importing of apkg files from external intents and make shared decks download via browser

Opening apkg files from external apps like chrome and gmail was broken, since recently they use a content provider instead of passing files directly. That was preventing us from being able to change over to handling shared decks using the standard browser. This PR fixes both issues. 

In the case where the decks are downloaded from AnkiWeb, a new `application/apkg` MIME type is used in the manifest to catch the file. Unfortunately in other cases like gmail, there's no way to exclusively catch apkg files, so I had to set the manifest to catch the generic binary file type `application/octet-stream`, and then check the file extension from within AnkiDroid. This is not optimal, but [unfortunately that seems to be the only way](http://stackoverflow.com/questions/14256916/open-custom-file-type-gmail-attachment-in-app).