Improve handling of small largeIcons in tablet ticker.

The tablet system bar is too short for a full notification
row, but it will show the largeIcon of a new notification in
its full frame, intruding into the main application content
rectangle a bit. This is a good thing---the notifications
really pop out at you---but it looks a little odd if the
largeIcon is actually *small*: the icon is vertically
centered in the larger (invisible) rectangle, making it look
poorly centered on the system bar.

This change will detect short largeIcons and center them in
the system bar's height instead. Additionally, it applies
the same ticker background all the way to the left
underneath largeIcons that are not full-frame (and therefore
have transparent regions).

Bug: 5708907
Bug: 5560485
Change-Id: I32af45632df164c58fb960110063dbaa08b5536c