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Currently the shelf navigation setting is supposed to be hidden on
clamshell-only devices. PowerManagerClient is used to detect when tablet
mode is unsupported but has not been accurately been detecting when
this is the case.

After discussion with UX, it has been decided to instead only show the
setting when in tablet mode. This will ensure that clamshell-only users
are not shown a setting intended for tablet mode users.

This change removes usage of PowerManagerClient and instead uses
ash::TabletMode::Get()->InTabletMode() to determine when the shelf
navigation setting should be shown in a11y settings.
ash::TabletModeObserver is also used so that when tablet mode is changed
the setting is shown in tablet mode and hidden in clamshell mode.

Bug: 1062213
Change-Id: I01a7c3946c557dc65eebd69b52b63240aa39461e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2197868Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Matthew Mourgos <mmourgos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768513}