libunwindstack: Set update mechanism

This CL sets the Update Mechanism to Manual.

R=qjw@chromium.org

Bug: 445311061
Change-Id: I3b5cce4ced8d01b91c8bc3ede826325d384e86df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/third_party/libunwindstack/+/7509829
Reviewed-by: Jiewei Qian <qjw@chromium.org>
Owners-Override: Jordan Brown <rop@google.com>
Reviewed-by: Patrick Meiring <meiring@google.com>