Fix three-line notifications.

To accommodate three lines as before, we now adjust the size
of text and icons to fit.

Also in this change:

 - snazzy little shadow below BigPicture's 1U block
 - restore contentText to BigPicture notifications
 - make overflow match other notification text (not so big)
 - fix text position jumps between 1U and expanded
   notifications across all templates

Bug: 6519374 (three-line notifications cut off)
Bug: 6494982 (text jumps)
Change-Id: I717ec2dde805c25e3b949641ca7e69e4d8fc86d2