WASHINGTON — President Donald Trump on Tuesday signed legislation and an executive order that he said will hold China accountable for its oppressive actions against the people of Hong Kong, then quickly shifted his speech in the Rose Garden into a campaign rally-style broadside against Democratic rival Joe Biden. The legislation and order are part of the Trump administration’s offensive against China for what he calls unfair treatment by the rising Asian superpower, which hid details about the human-to-human transition of the cornoavirus.