GitLab

- When user has only one desk, mode switching should be off and
the tab slider should be hidden.
- Add Ash unit tests to check visibility and existence of the
interactive component when the number of desks changes.

Bug: 1161243
Test: Manual test (see a video in crbug) and Ash unit tests
`ash_unittests --gtest_filter=ModeSelectionWindowCycleControllerTest.SingleDeskHidesInteractiveMode`

Change-Id: I32708306cd310d4075ee9bbb20991e3bad20531f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2603854
Commit-Queue: Cattalyya Nuengsigkapian <cattalyya@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840903}