doc: add instruction for installing Android SDK in COMPILING.md (#6593)
doc: add instruction for installing Android SDK in COMPILING.md (#6593)


ejona86/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,ejona86/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java