#2 Change static protocol strings to an enum in SipProvider

[Brent]

Currently:

/** UDP protocol type */
public static final String PROTO_UDP = "udp";
/** TCP protocol type */
public static final String PROTO_TCP = "tcp";
/** TLS protocol type */
public static final String PROTO_TLS = "tls";
/** SCTP protocol type */
public static final String PROTO_SCTP = "sctp";

but should be converted to an enum instead.

[Brent]

I've changed the fields themselves, but I still need to go through and change the references to the fields everywhere to use the actual enum values instead of Strings.

[Brent]

The same enum should be used in the class in bug #2875441.

There may be other classes defining these redundantly elsewhere, too.