[ntp footer] Add string for hiding footer.

spec: screen/4XzU435Z5KFgsQh

This is to make sure the string gets translated in time for M139. A
future CL will actually use it in the UI. This will replace the string
added at [1].

[1]https://chromium-review.googlesource.com/c/chromium/src/+/6639836

Change-Id: I3535991e38f906e686ff55abd8ebe03955d8b97d
Bug: 424872616
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6643846
Auto-Submit: Paul Adedeji <pauladedeji@google.com>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1474514}