Fix Chooser-to-editor transition animation

Add a scene animation - image thumbnail in Chooser to full-size image in
the editor - when the thumbnail is visible.
Finishing Chooser activity while the transition animation is running
produces an effect of "flashing" background (as the activity disappears
before the animation completed). Thus the call to finish the activity is
delayed until the activity is stopped (i.e. becomes fully invisible).

Fix: 231448720

Test: Use 'Files' app to share an image, select Edit in the Chooser,
 observe animation (optionally slow down the animation in the developer
 settings)
Test: The same as the previous test but in the split-screen mode.
Test: Slow-down animation, select "edit" in the chooser and cancel the
 animation mid-flight. The ChooserActivity pauses and resumes and
 remains overall functional (had to slow down the fade-out animation in
 the code). The animation in this case is broken but
 addressing this case is beyond the scope of this change.
Test: Use a mock app with the dialog-themed activity instead of the
system image editor; check that Chooser fades out and finishes in
background.
Test: Accessibility with talk-back on.

Change-Id: I982b9b2a8a049b5986dce521ee1abcd9ad83ba38