Support ALPN on Android 4.4+.

ALPN is the successor to NPN. When it's widely deployed we
can drop support for NPN altogether.

Verified manually on my Android device against https://gmail.com
(which uses ALPN) and https://twitter.com (which uses NPN only).

https://github.com/square/okhttp/issues/128
Merge pull request #440 from square/jwilson_0112_alpn

Support ALPN on Android 4.4+.