Rust module in APEX uses stub libraries across APEX boundaries

This change fixes a bug that rust module in APEX has access to private
symbols of a native library even when the native library is outside of
the APEX.

To fix this, the stub selection logic in the cc package is exctacted as
a function ChooseStubOrImpl and is used also in the rust package.

Bug: 190767845
Test: m
Change-Id: I5c4cbdd5d27f257ab329d9dadbcd87d41a87f46a