Allow ota_from_target_file to work entirely on directories

When building android, build system will first zip target_file directory
into a .zip file, and then invoke ota_from_target_files. We
ota_from_target_files can work directly on top of directories, we can
parallelize OTA generation and target_file zipping, making builds
faster.

Bug: 227848550
Bug: 277028723
Test: th
Change-Id: Id72bd7cca708af0e1bae2db754f39b27777cd601