Build jre_emul.jar with ijar to reduce size

Building jre_emul.jar as an ijar (jre_emul.ijar) reduces its size from 5.8M to 3.4M (2.4M reduction).

Unfortunately, this requires a Bazel build from https://github.com/bazelbuild/bazel/tree/master/third_party/ijar, so we'd have to make it an optional dependency like we do for protobufs.