consolidate protocol constants

Changed all internal use of NPN protocol identifier strings to use a corresponding constant in `Protocols`.

One last thing I'd like to is remove `OkHttpClient.setTransports(List<String> transports)` for its `setProtocols` counterpart, and also rename the `-Selected-Transport` header to `-Selected-Protocol`.

Our `Transport` interface only has 2 subclasses (basically spdy'ish or not), and I think the user intention is to select, or know about the protocol vs the transport impl.