Update GetValuablesDataManager API in autofill client.

The signature is updated from returning a reference to a raw pointer.
This is done to convey that ValuablesDataManager is not available on all
platforms e.g. iOS, so that calls in components/ make sure to handle the
case where the manager is not available.

Bug: 410795997
Change-Id: I5fc613c250389e46f5af1e6a4f32ab0c993e1f5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6469653
Commit-Queue: Norge Vizcay <vizcay@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1448364}