Moves java overview project to a subdirectory

java/pom.xml

java/run_greetings_client.sh

java/run_greetings_server.sh

java/src/main/java/ex/grpc/GreetingsClient.java

java/src/main/java/ex/grpc/GreetingsGrpc.java

java/src/main/java/ex/grpc/GreetingsImpl.java

java/src/main/java/ex/grpc/GreetingsServer.java

java/src/main/java/ex/grpc/Helloworld.java

java/src/main/proto/helloworld.proto