[mobar] Check ob focus when gaining form field focus

This fix, which drops the ff focus event while the ob is focused,
assumes that we'll get either a ff focus or keyboard vis change event
after the ob loses focus. In all likelihood we'll get both and the ff
focus + ob focus combination is transitory.

Bug: 448382678
Change-Id: I5d243f3c69b1e8a8aa2419e8b4dd4b57ef4816b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7038191
Reviewed-by: Tomasz Wiszkowski <ender@google.com>
Commit-Queue: Patrick Noland <pnoland@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529078}