Fix type registration for policy_prefs::kLastPolicyStatisticsUpdate

This is a functional no-op, but fixes the registration call to match
get/set calls in preparation for enforcing this in a subsequent CL.

Bug: 438680281
Change-Id: I54cafaed3df668f41a4ee4c80a5be21acb0e5971
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6845658
Auto-Submit: James Lee <ljjlee@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Victor Gabriel Savu <vsavu@google.com>
Commit-Queue: Victor Gabriel Savu <vsavu@google.com>
Cr-Commit-Position: refs/heads/main@{#1505946}