Implement "shared element" SS->LSS transition in screenshots

Workaround for b/190806800; in order to get the transition from
screenshots to the long screenshots activity to look the way we
want, we fake the shared element transition by determining the
destination for the preview in long screenshots, sending that back,
and running the actual animation in screenshots.

Bug: 183197533
Test: manual; visual verification in portrait and landscape
Change-Id: I16d462789ec0fa9422cc1924e6df9649b89b9055