okhttp: Make Headers package-private
okhttp: Make Headers package-private

There was no way to use it without touching internal APIs, and it should
never have been public to begin with as it serves no purpose to a user.

dapengzhang0/grpc-java,simonhorlick/grpc-java,simonhorlick/grpc-java,pieterjanpintens/grpc-java,elandau/grpc-java,simonhorlick/grpc-java,elandau/grpc-java,stanley-cheung/grpc-java,carl-mastrangelo/grpc-java,zhangkun83/grpc-java,elandau/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,zpencer/grpc-java,carl-mastrangelo/grpc-java,ejona86/grpc-java,grpc/grpc-java,carl-mastrangelo/grpc-java,zpencer/grpc-java,rmichela/grpc-java,pieterjanpintens/grpc-java,ejona86/grpc-java,grpc/grpc-java,elandau/grpc-java,zpencer/grpc-java,dapengzhang0/grpc-java,zhangkun83/grpc-java,grpc/grpc-java,pieterjanpintens/grpc-java,pieterjanpintens/grpc-java,zhangkun83/grpc-java,zhangkun83/grpc-java,stanley-cheung/grpc-java,zpencer/grpc-java,stanley-cheung/grpc-java,rmichela/grpc-java,dapengzhang0/grpc-java,rmichela/grpc-java,ejona86/grpc-java,simonhorlick/grpc-java,carl-mastrangelo/grpc-java,rmichela/grpc-java