Make module stubs publicly visible

Previously they were limited to frameworks/base so that they could be
combined into the "main" android stubs. However, limiting their
visibility is inflexible and unnecessary, and due to limitations in the
build system also makes it impossible to create rules for prebuilts of
these module stubs that set `prefer: false`.

This CL makes it possible to disable the prebuilts, which multiple
downstream branches would like to do.

Bug: 159902351
Test: m nothing (with prefer: false on prebuilts)
Change-Id: Id0eee4bf4e78f5dfddf6ad569e49719fefde658e