examples: Add google mirrored maven central to examples pom.xml to deflake kokoro. (#4813)
examples: Add google mirrored maven central to examples pom.xml to deflake kokoro. (#4813)

Add google mirrored maven central to examples pom.xml to deflake kokoro.
carl-mastrangelo/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,zhangkun83/grpc-java,carl-mastrangelo/grpc-java,zhangkun83/grpc-java,carl-mastrangelo/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,grpc/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,zhangkun83/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,zhangkun83/grpc-java,carl-mastrangelo/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,dapengzhang0/grpc-java