[Switch Access] Refactor constants file

Separating this out into its own changelist.

In order to cleanly and clearly express the preferences available,
we need to be able to append the Commands to the Preferences (as each
command can have a switch associated with it). The language did not
allow that with the previous construction.

Bug: None
Change-Id: I9166a91436b44479b6a656748190f412980fdd27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578238
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#653759}
4 files changed