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This patch shows two related photos together when device is in landscape
orientation.

Major changes:
1. Only show paired photos in landscape mode.
2. Use flex layout to dynamically add and show the secondary image.
3. If the primary image is fetched, it will be shown in the UI.
4. If the primary image is not fetched, then will ignore the secondary
   image, i.e. read from cache.
5. If the second image is fetched, it will be added to the UI if the
   screen orientation is landscape.
6. There is a 8px wide separator between two photos.
7. Only update the cache with primary image.
8. Paired photos share the same attributes.

Screenshot: https://screenshot.googleplex.com/9UirWBGxheYVqV4

Bug: b/157080996
Test: added new tests.
Change-Id: I25fb18104f317ffad629bbf7d1fcc5302806cf20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407176
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809702}