[ios] Fix AppBar button centering in landscape

This fix changes the offset applied to be zero in landcsape and -2 in
portrait. This is a follow-up of https://crrev.com/c/7768335 which
accidentally applied the offset when in landscape instead of portrait.

Additionally, the offset had to be applied by changing the buttons'
contentInset rather than changing the StackView's top and bottom
constraints, because the StackView is linked to layout guide
`kAppBarGuide`, and other views are constrained to it.

Fixed: 510284631, 502868239
Change-Id: Idaed7a581462e1a8c531fe2aa93d96932a27463b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7842812
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Scott Yoder <scottyoder@google.com>
Cr-Commit-Position: refs/heads/main@{#1629561}