Create richer notifications using available APIs.

On Jelly Bean and above, create a BigPictureStyle notification, which expands to
show the thumbnail of the current episode. On Honeycomb and above, use that
thumbnail as the largeIcon. Notifications on devices below Honeycomb are
unchanged.
Fix unsafe concurrent accesses in NowPlayingPollerThread.

Also, clean up some messy indentation.
Update notification cover art more aggressively.
Scale cover art for large icon notifications.