SequencedTaskRunner for OrchardSyncState should have more TaskTraits

[supermassive]

brave/brave-core#33971 (review)

Per Chromium guidelines, explicitly specify base::TaskPriority and shutdown behavior when creating thread pool task runners rather than relying on defaults. See OrchardSyncState::CreateSyncStateSequence() in components/brave_wallet/browser/internal/orchard_sync_state.cc.

Also tests which call SetupSyncState with base::SequencedTaskRunner::GetCurrentDefault() should be fixed somehow to always use OrchardSyncState::CreateSyncStateSequence()