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The music note icon is static. To avoid screen burn, we move the media
string view similar to the attribution.
1. Move `MediaStringView` to `AmbientBackgroundImageView` so that we can
   reuse the same logic to move attribution.
2. Move media string view in small steps on each refresh with in a fixed
   bounds.

Bug: b/168568042
Test: manual
Change-Id: Ieff99fbabb559139cb99cb2db808b65234af04d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437736
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813104}