Fix default centering logic in exo.

1) Do not center if the position when emulating x11 override redirect.
   This is used to tell  x11 WM not to manage the bounds, and exo should
   do the same.
2) Use the correct size when centering. The widget size may not be
   same as the content size if there is a pending configure. Use Visible
   bounds instead.

Bug: b/207430591, b/224381529
Test: covered by unittests
Change-Id: I3eb1ac3b009355be423d512861a537b855b15766
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3522460
Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/main@{#981422}