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In ambient mode, when user interacting with the background image by
mouse or gesture, lock screen contents (login pod and media control
view) need be shown on top for users to either do re-authentication,
or media control.

This CL implements a prototype of this view transaction by:
1. Show the lock screen contents, and
2. Fade-out the current shown image.

Bug: b/149246238
Test: manually.
Change-Id: Ie31c666cdfaddefc863ff8173dc185d73ef19178
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2110689
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757120}