Network: Use HttpURLConnectionFactory rather than OkHttp APIs

This is a refactoring with no behavior change.

The new class provides an abstraction layer to hide knowledge
about OkHttp-specific APIs. Logic from android.net.Network that
knew about OkHttp is moving into that abstraction layer.

This CL refactors android.net.Network to make use of this
abstraction layer instead of the tight coupling onto OkHttp
APIs. The class no longer imports any classes from okhttp
packages.

The values of mDns and mConnectionPool, which never change after
the initial call to maybeInitHttpClient(), are now set directly on
the AndroidHttpClient instance when it is constructed in that method.

Applications can overwrite getSocketFactory() and might depend on
that method being called (and the result used) every time a
connection is opened; therefore, for maximum app compatibility this
call was kept inside openConnection().

This CL is a prerequisite for introducing an additional frameworks
dependency on a richer API than HttpURLConnection.

Test: Build and install apk for FrameworksCoreTests, then run:
      adb shell am instrument -e class android.net.NetworkTest -w com.android.frameworks.coretests
Bug: 64021405

Change-Id: I2c73d260508ee20c6a40fd6e95e2d058d3ea2330