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When a new display is added during ambient mode, make sure that each
image view has the same images.

Prior behavior:
  * OnImagesReady internal display initializes -> {0*, _}
  * internal display receives immediate OnImageAdded -> {0*, 1}
  * external display is connected -> {0*, 1}, {0*, _}
  * OnImageAdded for image 2 -> {2, 1*}, {0*, 2}
    - external display does not switch image because none on deck
  * OnImageAdded for image 3 -> {2*, 3}, {3, 2*}

New behavior:
  * OnImagesReady internal display initializes -> {0*, 1}
  * external display is connected -> {0*, 1}, {0*, 1}
  * OnImageAdded for image 2 -> {2, 1*}, {2, 1*}

Initialize |PhotoView| with both images stored in |AmbientBackendModel|
to guarantee they show images in the same order. When the second
image is added, call |OnImagesReady| after |OnImageAdded| so that
|PhotoView| does not receive an immediate |OnImageAdded| after
being constructed.

BUG=b:173229098

Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Change-Id: I0f47d9a1b3748d74502080da9f0011e013897393
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547308
Commit-Queue: Jeffrey Young <cowmoo@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837306}