Seeking to put the best face on a difficult relationship with Beijing, Secretary of State John Kerry said Thursday that the United States and China could find ways to manage their differences and had more in common than not. Neither side wanted to fall into the “trap of zero sum competition,” Mr. Kerry said at the conclusion of an annual strategic and economic dialogue between top officials of the two countries. The array of topics with some areas of agreement — climate change, Iran, North Korea and Afghanistan — attested to the viability of the relationship, he said.