The Trump administration said Thursday it would allow new offshore oil and gas drilling in nearly all United States coastal waters, giving energy companies access to leases off California for the first time in decades and opening more than a billion acres in the Arctic and along the Eastern Seaboard. The proposal lifts a ban on such drilling imposed by President Barack Obama near the end of his term and would deal a serious blow to his environmental legacy. It would also signal that the Trump administration is not done unraveling environmental restrictions in an effort to promote energy production.