Refactor PersistentProto

This CL refactors PersistentProto by achieving the following things:

1. In persistent_proto.h, hide `ReadStatus` and `WriteStatus` in an
   internal namespace. Because PersistentProto users should not be
   interested in these low-level details. Actually, users never use
   `ReadStatus` or `WriteStatus` in the product code.

2. Use the helper classes in base/callback_list.h to replace the
   std::vector of callbacks.

3. Rename `OnRead` to `OnInit`. Also, update the class comment
   correspondingly.

4. Remove unnecessary usage of `base::WeakPtrFactory` in the code
   of PersistentProto users where the protos are owned by users.

This CL should not bring any noticeable difference.

Bug: b/259442843
Change-Id: I057dde214d0104190df3bc43b6a986afb19f6327
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5324507


Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: CJ Huang <chenjih@google.com>
Cr-Commit-Position: refs/heads/main@{#1267309}