This change forces metadata retreiver threads to background priority.
Uses an inner class to encapsulate the priority change so that it
automatically restores priority when returning to the client.
1. mRetriever was initialized twice: one in the constructor and the other in the setDataSource
2. cached the mode so that setMode can still be called before setDataSource.
mRetriever won't be initialized until data source is set; thus we have to do this
