Long screenshots: allow capture in reverse direction

Captures in the opposite direction if an edge is detected
early. This allows more useful behavior in "transcript"
style UI where the content grows upward and the user is
typically scrolled near the bottom edge.

Bug: 179378294
Test: manually, in messages or chat
Change-Id: I5c765b4e103990f428fcd24e26bd09536d613d96
1 file changed