Always take an activity screenshot when pausing.

http://ag/261732 introduced an optimization where we only take
the activity screenshot if is is not the activity we took a
screenshot for last. This causes us to display a stale screenshot
for activities that change their display content (using fragments)
without going through the pause/resume cycle. It should be safe
to always take a screenshot when we are pausing since the code
path is only called once per pause/resume cycle.

Bug: 18682160
Change-Id: Ie5c43cfd806286808af4233c5917ae7071908ebb
(cherry picked from commit eacdf2ce)