[Android][Signin] Avoid resizing sign-in UI when keyboard is shown

When the sign-in UI (e.g. fullscreen sign-in) is started when a keyboard
is shown on the screen, the sign-in layout can be resized to adapt
to the keyboard, then resized again to take the whole available space.

As the sign-in activity doesn't contain keyboard-related features and
there's no plan to add any in the near future, this CL sets the
windowSoftInputMode to adjustNothing to avoid the visual glitch.

Bug: 414419626
Change-Id: I1ef575b59674e52ecb1ecfb8dbc202f4d6f76487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6546837
Auto-Submit: Menghan Yang <myuu@google.com>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460776}