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It was not handled that when a media session info is changed from paused
to playing, the string animation will not restart.
This patch simplifies the logic, when the media is paused, it will only
set the visibility to false, but not stop the animation, when the media
resumes, it is still animating.

Bug: b/173804133
Test: manual and modified test
Change-Id: Ic2a935b2bc52d8379189395dd3fd1213eefd48a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2573682Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834001}