Revert "[Settings] Enable the single-activity mode by default"

This reverts commit df77cbacb9476b0b6d3e75cc34b5d127985e79c3.

Reason for revert: Caused regressions b/375159749

Original change's description:
> [Settings] Enable the single-activity mode by default
>
> Bug: b/356743945
> Change-Id: If9c67a84315149615e95c9f98bd2bebda2a120b0
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5869178
> Reviewed-by: Sirisha Kavuluru <skavuluru@google.com>
> Commit-Queue: Shuhei Takahashi <nya@chromium.org>
> Reviewed-by: Theresa Sullivan <twellington@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1362307}

Bug: b/356743945, b/375159749
Change-Id: Ia355eabc754d3b523d126cf292ecc43eeee9524d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5976501
Commit-Queue: Sirisha Kavuluru <skavuluru@google.com>
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1375338}