[android] Move most ChromeBackupAgent pref work from C++ to Java

There should be no behavior change. Before this CL,
ChromeBackupAgentImpl read/set/observed PrefService data in
C++ [1][2]. This CL moves most of the logic to Java, for 2 reasons:

a) Less indirection: PrefService has good Java bindings, which can
   be used directly, making the code simpler.
b) The JNI abstractions weren't great: getAccountSettingsBackupName()
   was very sync-specific, whereas getBoolBackupNames() was very
   generic (...but then would have to be used in sync-specific ways
   in  crrev.com/c/5412214).

Unfortunately, some of it needs to stay in C++, due to lack of Java
bindings. The CL keeps thin JNI wrappers around PrefService for the
following:
* Reading/writing dictionary prefs. According to [3] there's a chance
  this will be implemented in the future. At that point the
  dictionary serialization/deserialization could be done by in Java
  too (% maybe migrating data serialized in the "old way").
* Flushing pref writes, i.e. CommitPendingWrite(). This would be easy
  to expose, but it's also a very niche use case, so maybe not worth it.

[1] https://source.chromium.org/chromium/chromium/src/+/main:chrome/android/java/src/org/chromium/chrome/browser/ChromeBackupAgentImpl.java;l=732-746;drc=4a4b6563f247290b03973958bda6ba99f421a00d
[2] https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/android/chrome_backup_watcher.cc;l=15;drc=c6025b06e27049e0b62257398027c5a1e28822cb
[3] https://groups.google.com/a/google.com/g/chrome-sync-dev/c/sd4tItY9J5k

Bug: None
Change-Id: If87f8e3f154e1945e3bdd2540c622154f61d80e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5419139


Reviewed-by: Menghan Yang <myuu@google.com>
Auto-Submit: Victor Vianna <victorvianna@google.com>
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1283142}