Force pending transactions to flush before screenshot.

Following 14e54ba747 (ag/1043009) we need to push an empty
synchronous transaction if we want to ensure all previous
transactions have occured before taking a screenshot. In
light of Bug 7552304 it seems wise to do this before screenshoting
applications.

Bug: 27098060
Bug: 7552304
Change-Id: I6d7dfbe634a288c55449d2f1d6fbbfc13bab08ad