focus-mode: Update strings for DND and first time

This CL updates the strings for the DND notification as well as first
time user flow for the focus panel. Also marks them for translation.

Bug: b/288975135
Test: manual
Change-Id: I29028f99d71cc36f9d4fb15bb03839195b799170
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5608615


Commit-Queue: Hongyu Long <hongyulong@chromium.org>
Auto-Submit: Richard Chui <richui@chromium.org>
Reviewed-by: Hongyu Long <hongyulong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312200}