[CrOS Cellular] Focus on UI elements in eSIM Flow

Updates "add eSIM" dialog UI to focus on the scan code using camera
button in the activation code page when camera is available or to
focus on the text input field when the camera does not exist.

Bug: b/322872567
Test: Deployed to DUT and verified the fix
Change-Id: I86116bbc48db89534c9d7417a57de6ffde6ab2c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5308364
Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Commit-Queue: Theo Johnson-kanu <tjohnsonkanu@google.com>
Reviewed-by: Gordon Seto <gordonseto@google.com>
Cr-Commit-Position: refs/heads/main@{#1285408}