Fix duplicate events for auralinux accessibility event recorder

When a test uses continuations to call the "go" function multiple
times a new instance of the event recorder is created for each
call. This results in the ATK event hooks being added for every
time without the old ones being removed.

This fixes it array of event listeners shared amongst all instances of
the event recorder and not adding the listeners if they already
exist.

Bug: None
Change-Id: Ie09212f7b44113ad7c0aeeba963615fd4b8607e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1840012
Reviewed-by: Martin Robinson <mrobinson@igalia.com>
Commit-Queue: Jessica Tallon <jtallon@igalia.com>
Cr-Commit-Position: refs/heads/master@{#702828}
1 file changed