Optimize large preview from the screenshot notification.

Crop out a notification_panel_width by notification_max_height
section of the screenshot for the preview.

This will only save about 25% of the bytes on a phone-like device over
the existing square crop, but on a tablet-like device the savings will be
more significant: above 75%.

Bug: 16631778
Change-Id: I05d4d402e44f7700f17b8b97018f6c00c3b0196d