PHNOM PENH, Cambodia — The United States has lifted a four-year arms embargo on Cambodia after announcing a resumption of the annual Angkor Sentinel military exercises in a sign that relations between Washington and China’s tiny but key regional ally are on the mend. Details of the resumptions were finalized by the Indonesia-based U.S. Mission to the Association of Southeast Asian Nations on the sidelines of last month’s ASEAN Leaders Summit in Kuala Lumpur, Malaysia, between President Donald