Android: Unread Counts for Reading Overlay

An initial framework for the reading-progress part of the overlay widget.  Indicates progress thru unreads as you read.

Notes:
* I couldn't find any widget like the circular progress indicator used in the iOS overlay, so subbed in the familiar and well-understood unread-count widget used everywhere else on the site/app.
* This patch enables this new part of the overlay for feed reading and blurblog reading.  I will add support for folder reading after context switching back from critical bugs.