[side-ui] Refactor SideUiContainers for multiple container support

- New interface method getSideUiId
- Replace mSideUiContainer with a list
- Replace mRight/Left anchor container with a map
- Refactored internal implementation to loop through list/map

No functional/behavioral change is made since there is a single
SideUiContainer.

Bug: 478338737
Change-Id: I3f2d855f90ea3724a1e2a6c924927526bf529bf2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7875451
Reviewed-by: Linyu He <linyuh@google.com>
Reviewed-by: Gazal Agarwal <agazal@google.com>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Theresa Wellington <twellington@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1638114}