Remove NPN support from Android's OkHttp Platform class

The Platform class is used by Android's built-in HttpsURLConnection
instances. We disable SPDY and HTTP/2 entirely so this should be a
no-op for apps using Android's HttpsURLConnection.

We run OkHttp's JUnit tests with this version of Platform. The OkHttp
tests do test SPDY and HTTP/2. ALPN supercedes NPN. The NPN support
in Android's socket implementation appears buggy and causes test
failures. Dropping NPN enables us to work around issues in those
tests.

There has been a pull request to drop NPN support generally from OkHttp
upstream: https://github.com/square/okhttp/pull/1201

See also:
https://www.imperialviolet.org/2013/03/20/alpn.html
https://groups.google.com/a/chromium.org/forum/#!topic/security-dev/eJVxCZ92vvg

Bug: 18705877
Change-Id: Iea58ba88e79cf1ac8cf08b4be900d27247f219e1