Move apache specific portions of android.net.http to external/apache-http.

We continue to compile external/apache-http into ext.jar. This contains
a few changes apart fom the classes moving around :

- Makefile changes to build docs and api-stubs for now. A future change
  will revert these changes and remove these classes from stubs and
  docs.
- Hardcode event IDs in legacyerrorstrings to avoid a dependency between
  the frameworks and apache. These strings are on their way out and will
  never change anyway.
- Remove imports due to {@link} tags and use {@code} instead.
- Remove an accidental(?) dependency on apache commons code that's a
  part of apache-http.

bug: 18027885

Change-Id: I51cd038d846ec7d02c283a4541b10a6a9cf62ecf