Move dismiss animation into ScreenshotView

Lets the view handle more of its own logic around dismissal,
rather than needing to call back through ScreenshotController. Now
just calls back once the dismissal is complete. This will make
integrating the swipe dismissal code simpler.

This also ensures that the view is reset and the service connection
is ended after dismissing for any reason.

Bug: 157223526
Test: manual (ensured the same behavior from dismissing through
various means and if taking a second screenshot while the UI is
already up).

Change-Id: I7e15a9312a0b1c981733b70be24f0f1b22721d47