Add (empty) default implementations for PrefStore::Observer methods.

This is a small preparation for changing the signature of
OnPrefValueChanged to take a std::string_view. Currently, there is a
third-party implementation of PrefStore::Observer inside libaddress.
Adding default implementations lets us remove that method override
from libaddress and makes changing the function signature possible
in a single (follow-up) CL.

Bug: 349741884
Change-Id: I15d460aaf62a13519c93de78c932d1acdd53fddd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5662299
Auto-Submit: Jan Keitel <jkeitel@google.com>
Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Commit-Queue: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1320241}