Move keymap logic into `EiKeymap`.

Much of this logic will also be needed by `EiSenderSession` in order to
inject text events, so it makes sense for it to be precomputed when the
keymap is loaded and placed somewhere both classes can access it.

Change-Id: I485751cffceb59b3466b6991da2647b7ee43fc59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6876369
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1506824}