Add ALPN support to SSL socket factory

This adds the ability to use Application-Layer Protocol Negotiation
(ALPN) through the SSLCertificateSocketFactory. ALPN is essentially
like Next Protocol Negotiation (NPN) but negotiation is done in the
clear. This allows the use of other protocols on the same port (e.g.,
SPDY instead of HTTP on port 80).

Change-Id: Ie62926b455e252c4c98670bbbecc1eb5c6f13990