[Demo Mode] Set string length limit on the user input on the
preferences page

Set the string length limit to 256 bytes on both the retailer name and
the store number. We disable the continue button when the user input
is beyond this limit. It is to prevent a malicious use, and ensure it does not use too much storage or break Torus based on the 10240 byte
limit.

Bug: b/289436065
Test: browsertest
Change-Id: I39302868d84c59f5cfe07ce5dea6b10efd9cd116
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5270345
Commit-Queue: Haifan Wang <wanghaifan@google.com>
Reviewed-by: Li Lin <llin@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiqi Ruan <xiqiruan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1259646}