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This CL contains the following changes to the a11y_page:
* Updating the Captions button icon to use the external icon for MacOS and Windows.
* Rearrange code so that there is only one default route path and place everything
  except for templates with their own route path in the default route.
* Move the Captions button inside the settings-animated-pages. This fixes a focus
  glimpse when tapping on the Captions button using a touch screen device.
* Move the Captions subpage template below all of the buttons.

Bug: 971264
Change-Id: I354373f5715153fb3a89745dc88e12dedde80020
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1799304Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Evan Liu <evliu@google.com>
Cr-Commit-Position: refs/heads/master@{#697308}