Hookup CSS prefers-reduced-motion for Android

This CL hooks up ui::gfx::Animation::UpdatePrefersReducedMotion for
Android, and adds a ContentObserver in BrowserAccessibilityStateImpl to
track changes to the setting.

Test: Manually verified functional on Android
Bug: 722548
Change-Id: I2e8457a70f1f208b2a6479f8569e5bfe9aaac8c5
Reviewed-on: https://chromium-review.googlesource.com/c/1437996
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629346}
9 files changed