Fixed corner-case scenario where a screenshot is finished after the share

notification is sent.

Prior to this change, if a screenshot finished after the share
notification was sent, it would replace the share notification with a
progress notification, and the share notification would never be sent
again.

Also improved the test cases that automatically generate a screenshot
but don't use it to wait for the screenshot to finish before proceeding,
otherwise it could cause a future test to fail (if the screenshot is
finished after the initial test is completed).

Change-Id: I6e2a6549ebb48e5bebf5aa78d1bda94404c1812b