dnd: Ungrab events before snapping drag actor back

Inside this._ungrabEvents() we ungrab the pointer or the touch sequence.
Since touch sequences are no longer valid right after the touch i.e. the
drag ended, we really shouldn't access them later after the snap-back
animation finished.

Instead, simply ungrab the events before starting the animation to make
sure the sequence ungrab happens either during the touchpoint is still
on the screen or while handling a TOUCH_END event.
4 jobs for !528 with dnd-small-fixes in 37 seconds (queued for 3 seconds)
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failed #289741
check_commit_log

00:00:37

 
  Build
skipped #289742
build
 
  Test
skipped #289743
test
skipped #289744
test-pot
 
Name Stage Failure
failed
check_commit_log Review
 * [new branch]      dnd-small-fixes -> origin/dnd-small-fixes
Checking out 9fc0e8bd as dnd-small-fixes...

Skipping Git submodules setup
$ ./.gitlab-ci/check-commit-log.sh
From https://gitlab.gnome.org/GNOME/gnome-shell
* branch master -> FETCH_HEAD
Missing merge request or issue URL on commit 9fc0e8bd
ERROR: Job failed: exit code 1