Isolate OkHttp from frameworks/base.

Introduce a new class libcore.net.http.HttpURLConnectionFactory that
offers a subset of OkHttpClient APIs, suitable for use by Android
framework code as an abstraction layer to avoid tight coupling onto
OkHttp.

Some Android specific classes from android/ have also moved into
that package, with package private methods exposed via new helper
classes that remain in the OkHttp package.

The code added/moved to libcore.net.http remains in the
external/okhttp git repository because unlike libcore itself,
it has compile time dependencies onto okhttp.

Bug: 64021405
Test: libcore.java.net.URLConnectionTest

Change-Id: I85b7082ebb55b9ae000a4b593058d80b889263fd