Close screenshot process on user switched

Currently, we keep the process up even if the user switches,
meaning that in some cases (if the user is switched while the
screenshot UI is up) we will save images to the wrong profile.
This change makes ScreenshotHelper listen for user switches and
close the screenshot service, so that a new screenshot is
guaranteed to be constructed with the correct user's context.

Bug: 170474245
Fix: 170474245
Test: manual -- verified bad state occurs if user switches within
the timeout period, ensured that screenshots work immediately
after switching with this change.

Change-Id: I9d32d0928e6c2bda161d04555438d0dd7afef0ba