Close the device gracefully when cherrypy exits

When the cherrypy server exits, remember to close the touch device
gracefully. This prevents any local/remote zombie processes.

BUG=chromium:443539
TEST=Manually execute webplot.py on a chrome os machine, and press
ctrl-c to interrupt the server. Confirm that there are no evtest
and ssh zombie processes.

Change-Id: Ib8bd927cf78a4cb620f021298ca2cb392f9efab5
Reviewed-on: https://chromium-review.googlesource.com/241366
Reviewed-by: Charlie Mooney <charliemooney@chromium.org>
Commit-Queue: Shyh-In Hwang <josephsih@chromium.org>
Tested-by: Shyh-In Hwang <josephsih@chromium.org>