[Origin] Gate BraveProfilePolicyProvider on BraveOriginPolicyManager init
`BraveProvider`'s first `RefreshPolicies` is triggered by
`AdBlockOnlyModePolicyManager` (initialised at process start), well before
`BraveOriginPolicyManager` (lazily initialised by `BraveOriginServiceFactory`)
is ready. The resulting bundle lacks `BraveRewardsDisabled`, but the
default `IsInitializationComplete` lets `PolicyService` report ready
anyway. `AdsServiceImpl::MaybeStartBatAdsService` then evaluates a stale
managed pref store and bat-ads starts despite the policy.
Override `IsInitializationComplete` to additionally gate on
`BraveOriginPolicyManager::IsInitialized()`. Upstream unit tests
(`ProfilePolicyConnectorTest`) bypass our factory bootstrap, so add a
`SetExpectedToBeInitialized` flag set from `BraveOriginServiceFactory`'s
ctor; the override short-circuits when unset (desktop/Android only --
iOS doesn't run those tests).
Resolves: https://github.com/brave/brave-browser/issues/54962