ChromeVox: Allow gestures in the tutorial.

To fix this issue, this change:
1. Only uses menu key overrides if the current range is or is a
descendant of a menu item.
2. Ensures that swiping left/right in the panel still works for the
panel search menu by making the text field a child of a menu item.
3. Adds a test to confirm correct behavior in the tutorial.

Fixed: 1183892
AX-Relnotes: Touch gestures can be used to interact with the tutorial.
Change-Id: Ia5d6517de6dea152a5074a8966217da23917ea3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2768298


Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864888}