BigPicture notifications for screenshots.

Needed a little extra help from the builder to make this
happen (like the title, you might want a different largeIcon
in the expanded view if---as in this case---it is redundant
with the big picture).

Needed to fix BigPicture template to reveal actions.

Bug: 6289107
Change-Id: I19dc62e693ee2c87cbe82f7327fe9115768b4102