Fixing issue where screenshot notification remains non-dismissible.

- Ensure that we start the screenshot as a foreground service to reduce
  likelihood that it is killed while taking a screenshot.
- If the screenshot process times out or gets killed for any reason, 
  ensure that we update the notification with an appropriate error 
  message.

Bug: 27389179
Change-Id: I5007bda95538044bc753e4ceffd2f59a069c857b