[DevicePage] Add pointing stick acceleration setting

This CL adds a new setting for pointing stick acceleration, with a
corresponding slider on the pointers subpage. If the flag is disabled,
changes to the equivalent mouse setting are copied to the pointing stick
setting as well. See go/cros-pointing-stick-settings (Googlers only,
sorry) for the design doc.

Bug: 1114828
Test: Check the toggle changes acceleration
Change-Id: If9dd9cbcffd064ab897abcbe551990e2862e08ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548687
Commit-Queue: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832963}