Add java8 tests and build flags

[deannagarcia]

• Adds a java_opts.bzl file that creates a set of common java options and creates macros for protobuf_java_library and protobuf_java_export that automatically use those options
• Adds a java8 test
• Updates docker images for other java tests

[mkruskal-google]

Nice, good to see this infra still works! :)