Getting elements, querying length or copying elements
are now const functions.

Drive-by fix: Noticed a few more getters that should be const.
Add a comment to ArrayList functions that are static functions. 
BUG=

Change-Id: I5de1aed97510dea4e47cb974b3259da51ae663af
Reviewed-on: https://chromium-review.googlesource.com/467249Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44372}