[Phone Hub] Fix Web GAR issues - provide learn more aria label for TTS.

Bug: b/232199523
Test: Use crtl+alt+z to enable TTS and verify that we can hear "Learn more about Phone Hub"

Change-Id: Ie1b65f648e0851a99fa67a1abc08cfe2f677d3b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3694879


Reviewed-by: Jon Mann <jonmann@chromium.org>
Commit-Queue: Mavis Hsu <mavishsu@google.com>
Cr-Commit-Position: refs/heads/main@{#1012849}