By Gram Slattery and Susan Heavey WASHINGTON (Reuters) – A standoff over U.S. immigration policy stalled movement on a $1.66 trillion government funding bill in the Senate on Thursday, raising the risk of a partial shutdown of federal agencies just days before the Christmas holiday. Progress on the bill — which includes $44.9 billion in wartime aid for Ukraine and bans the use of Chinese-owned social media app TikTok on federal government devices — halted after conservative Republican Senator Mike Lee introduced an amendment meant to slow immigration. The amendment would require the United S…