Don't take app screenshot in minimize docked stack.

When the docked stack is minimized its app windows are cropped
significantly so any screenshot taken will not display the apps
contain. So, we avoid taking a screenshot in that case.

This situation is going to be improve some in O when we switch
recents to also use saved surfaces in which cases it will be able
to display the last app content the user saw vs. just the app's
background color.

Bug: 29830173
Change-Id: I003aa074126ddcb57d5ff6dda756293146646196