[iOS contextual panel] Fix Contextual Panel Entrypoint accessibility bug

Fix bug where sometimes the contextual panel entrypoint would be skipped
by voiceover due to the timing of when accessible elements were updated
in the location bar and the entrypoint still being hidden. Changed to
entrypoint taking care of its own accessibility status.

Bug: b:358435666
Change-Id: I28376189d4909e484acf0b6e41c837711a897c51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5776552
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Nicolas MacBeth <nicolasmacbeth@google.com>
Cr-Commit-Position: refs/heads/main@{#1340422}