[chromecast] Allow toggling magnification by triple-tap.

If the magnification gesture is enabled, the AccessibilityManager will
listen for triple-tap events and turn on (or off) the
PartialMagnificationController.

Bug: internal b/110532350
Bug: internal b/110481596
Test: manual
Change-Id: I3e1ed53fddb1c525c0cc3b238e243f59b737323d
Reviewed-on: https://chromium-review.googlesource.com/1125038
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573468}