Post a silent notification if screenshot is dismissed

Replicates the old behavior of being able to use the notifications
shade as a "to-do" area for screenshots. If the screenshot is
dismissed explicitly or by timeout (i.e., not by tapping the share
or edit buttons), we post a silent notification.

Bug: 156343422
Fix: 156343422
Test: manual
Change-Id: Ie2c1db4212e7332c5fba2e93c3d1d4d5e0c99a25