[OOBE] Conditionally hide subtitle on welcome

Fix: b:283986296
Change-Id: I2599a01b630fe6ddc8eab569977feaff47e0c27b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4556909


Reviewed-by: Danila Kuzmin <dkuzmin@google.com>
Commit-Queue: Simon Ziegltrum <ziegltrum@google.com>
Cr-Commit-Position: refs/heads/main@{#1148096}