testing: Move API to type text into shared location

This change moves an API to simulate keys being pressed into a shared
location so that it can be used by multiple tests.

Bug: b/40286410, b/338644695
Change-Id: Ib61fa1cc3396f199e5a9ccf252a9727a29353029
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5623431


Reviewed-by: Nikhil Nayunigari <nikhilcn@google.com>
Commit-Queue: Chad Duffin <chadduffin@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313722}