Fix backups and change to apkg

Make a number of changes to the backups:

 * Backup every time DeckPicker is successfully opened if more than 5 hours have elapsed since last backup
 * Use apkg backup instead of copying raw file
 * Do backup in the background using a new thread, showing a toast when it starts instead of a dialog
 * Increase default number of backups from 3 to 8
 * Show the usual restore dialog instead of forcing restore when RuntimeException on opening collection