apache-http: Fix stubs generation.

- We're asking doclava to generate stubs for all apache http packages.
  We must therefore point it to all apache-http sources (including the
  handful that live in frameworks/base).
- Stop asking for stubs for android.net.*. These classes are still
  public frameworks API and their stubs will be present in android.jar.

Change-Id: I7f1b7dc41988a896eef75c53689c7ca56bdec4a7