Add completion handler for ScreenshotHelper

The Next-Generation Assistant can call SysUI to request a screenshot.
Adding an indicator that a screenshot has completed successfully enables
the Next-Generation Assistant to show the assistant UI again once the
screenshot has completed.

Bug: 137201206
Bug: 131082115
Test: Wired up SysUIG's NGA integration's touch outside handler to call
take screenshot, confirmed it behaved as expected when a touch outside
occurred.
Change-Id: Ie83af6c9dd8e93b3e18a820d79c02125a19bd62a