GestureConverter: Stop flings with fake fingers

Currently, the only way to stop a fling (the scrolling that occurs due
to "momentum" after you've finished a two-finger swipe on the touchpad)
is to put two fingers down again and move them slightly. This is an
awkward gesture, compared to most platforms where you just touch the pad
again and scrolling stops.

The trouble is that there is currently no API by which we can tell apps
to stop a fling. Instead, make a fake finger at the current cursor
location, then immediately cancel it. This stops flings without side
effects (e.g. activating UI elements, inking in drawing apps, moving the
insertion point in text fields, etc.) in every app I've tried it in [0],
except for a number of issues in Google Docs, Sheets, and Slides (both
the Android apps and when opened in Chrome). We'll have to talk to the
relevant teams about this. Follow-up CLs will make an API change to make
it easier to distinguish these fling stops from actual clicks.

[0]: Calendar, Chrome [1, 2], Docs, Files, Firefox Beta [1, 2], Gmail,
Google, Keep, Maps, News, Photos, Play Store, Podcasts, Settings,
Sheets, Slides, Word, YouTube
[1]: https://www.longestjokeintheworld.com/
[2]: https://codepen.io/caraya/embed/ZyQMWd

Bug: 281106755
Test: start flings in a number of apps [0], touch the touchpad, and
      check that the fling stops without causing other side effects
Change-Id: Ie8504dc047ee23b5524d5372f65adfaa75a76cd8