Replace interal okhttp APIs

Connectivity is becoming a mainline module in S but mainline
modules are not allowed to use non-formal APIs. Thus, replace
internal okhttp APIs to stable libcore APIs which are created for
using HttpURLConnectionFactory.

Bug: 182238821
Test: atest FrameworksNetTests
Change-Id: I56ba1b9e6e94f9c6519c3f1c8f0c5993fccbe185