feat: re-enable Java example of using pre-built protoc

[alexeagle]

Also serves as a reproduction for protocolbuffers/protobuf#19679

Testing:

% bazel run java       
INFO: Analyzed target //java:java (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //java:java up-to-date:
  bazel-bin/java/java
  bazel-bin/java/java.jar
INFO: Elapsed time: 0.089s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/java/java
message: "Hello World!"

[alexeagle]

CI failure indicates an attempt to compile C++ code from source, which doesn't work on every Bazel user's machine (Bazel doesn't ship with a hermetic toolchain, and without one you can't reliably build)

 Building libfoo_proto-speed.jar (1 source jar) failed: (Exit 1): DUMMY_GCC_TOOL failed: error executing CppCompile command (from target @@abseil-cpp+//absl/time/internal/cctz:time_zone) tools/toolchains/DUMMY_GCC_TOOL -MD -MF bazel-out/k8-opt-exec-ST-b7c63d4a17f4/bin/external/abseil-cpp+/absl/time/internal/cctz/_objs/time_zone/time_zone_fixed.d ... (remaining 11 arguments skipped)