Don't maintain separate instance of Collection in AnkiActivity

Only store one instance of Collection in AnkiDroidApp so that we can manually reopen the collection without using the AnkiActivity Loader, and still have AnkiActicvity.getCol() return the most recently opened Collection.

Fixes this bug for instance:
http://ankidroid-triage.appspot.com/view_bug?bug_id=4559535838593024