Revert "cros: Merge part of TabletModeClient with TabletMode"

This reverts commit 5d46f946229cd775e25ad46579315e628c09c943.

Reason for revert: Speculative revert: TopControlsSlideControllerTest fails frequently after this patch.

Original change's description:
> cros: Merge part of TabletModeClient with TabletMode
>
> - Make ash::TabletMode a concrete class that holds a bool of whether
>   tablet mode is enabled and notifies observers when its changed;
> - Make ash::TabletModeController an ash::TabletMode::Delegate to
>   provide real state and test support;
> - Move tablet mode observing part from TabletModeClient
>   into ash::TabletMode. This leaves TabletModeClient only
>   to track and enable mobile behavior for tabs.
>
> Bug: 974238
> Change-Id: Ifda13f72a5016d62439bf96087a5fae3be9df50d
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1665553
> Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#671340}

TBR=xiyuan@chromium.org,sky@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 974238
Change-Id: I71d01fd8bc56c7d8e45baf8c0a974b93dcd6cb38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1672855
Commit-Queue: Huanzhong Huang <huanzhong@chromium.org>
Reviewed-by: Huanzhong Huang <huanzhong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671635}
57 files changed