[iOS] Prevent Voiceover from reading behind scrim during search

Before, when the scrim was up, dimming the results view, Voiceover
would still read the dimmed elements. Now, as part of putting up the
scrim, the table view marks all its subviews as accessibilityHidden,
so they don't appear in Voiceover. The opposite happens when the scrim
is removed.

Bug: 981343
Change-Id: Ibfb9d37f64c30f75fbe6d05633c7e6f4790f3713
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1826888


Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#700296}