Add corner flow for screenshots

Instead of showing a notification, the screenshot animates into the
bottom left corner of the phone. Tapping on it leads to the same
intent as tapping "Edit" on the notification would. The screenshot
disappears after 8 seconds.

Bug: 137153302
Test: adds feature behind flag, tested that behavior remains the same
if flag is false

Change-Id: I4f48ba7de36f298bd6a66439ea6885fc92a8cb9c
2 files changed