Include the okhttp.jar when compiling against internal classes

If LOCAL_SDK_VERSION is unset we were compiling against
core-libart but not okhttp.

okhttp should be an implementation detail but
android.net.http.HttpResponseCache implements
com.android.okhttp.OkCacheContainer.

The OpenJDK 8 version of of javac now requires all interfaces of
implemented classes be made available at compile time even with
-source 1.7 -target 1.7.

This is not an issue when compiling against the android.jar because
the implementation of HttpResponseCache in the android.jar does not
implement com.android.okhttp.OkCacheContainer.

Bug: 23099154
Change-Id: I3fa1d0f73535b396623fed953cfc71189b797849