FaceGaze: Update UI from the extension

This change adds the logic that allows FaceGaze to control the
content of the bubble UI. Whenever there are newly recognized
gestures (and resulting actions), FaceGaze will display them in the
bubble UI. Tests have been added to confirm this behavior.

For usability, we also ensure that the bubble text is cleared if no
gestures are recognized for a short period of time - this ensures that
the bubble text is always up-to-date.

Bug: 367774352
Change-Id: Iea4540250ae77669e8d4dc77fbe174588ddb3e34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5875411
Reviewed-by: Amanda Lin Dietz <aldietz@google.com>
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1358954}