paygen: Generate the payload hash and metadata hash together. Due to recent changes in delta_generator, payload hash and metadata hash are calculated together, calling it twice are just doing repeated work. BUG=None TEST=./paygen_payload_lib_unittest Change-Id: I62c6a3621e2da882a5acd8209c17686152810806 Reviewed-on: https://chromium-review.googlesource.com/310226 Commit-Ready: Alex Deymo <[email protected]> Tested-by: Sen Jiang <[email protected]> Reviewed-by: Matthew Sartori <[email protected]> Reviewed-by: Alex Deymo <[email protected]>