[Android] Set syncable pref for the last plus address filling time.

This pref will be used to attach additional data to the plus address
HaTS surveys.

Bug: 348139343
Change-Id: I04ecc02f8c374a3bb0e0d31dceaa78e83671c2fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5974272
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Timofey Chudakov <tchudakov@google.com>
Cr-Commit-Position: refs/heads/main@{#1378237}