desks: Add 4-finger horizontal swipe to switch desks.

This implements 4-finger swipe australian mode (swipe left to go right).
I looked at making this work if we switch to traditional mode in settings
but that seems to affect scroll at the event level but not for stuff
other than page scrolling (ie tab scrubbing).

We can add it in the future manually but that requires some plumbing
through the event/devices apis.

Test: added tests
Bug: 970381
Change-Id: Iff46d4ef17442e2a302d0a80c612641df3c0ffdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719669
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681522}
8 files changed