snap-groups: Remove `divider_position_` during transition

This removes the need for `divider_position_` during tablet <->
clamshell transition, before `split_view_divider_` is created. This
will allow us to move `divider_position_` to `SplitViewDivider`.

This also modifies `GetClosestFixedDividerPosition()` to take in
`divider_position` as an argument, since we may need it to get the
closest divider position before `divider_position_` as a member
variable is set.

Test: existing
Bug: b/318862377
Change-Id: If8f36d6277a12e7d217cff20c1367735af9593a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5177274
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Sophie Wen <sophiewen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1245065}