Add an enable_brave_stats_updater buildflag that defaults to
!is_brave_origin_branded, following the pattern used by other features
(enable_brave_ads, enable_brave_rewards, etc.). On Brave Origin builds,
exclude brave_stats_updater.cc/.h and its tests via GN, guard callers
with #if BUILDFLAG, and put a static_assert in the header so any
accidental include fails with a clear error.
Resolves https://github.com/brave/brave-browser/issues/55595
Brian R. Bondy
d15f720f3c