Add ApexModule interface for APEX-aware modules

ApexModule is the interface for APEX-aware modules. The module type apex
uses the interface to get APEX-specific information from other modules,
such as the list of APEXs that a module should be built for.

A module that is included in an APEX will be built specificaly for the
APEX. This is especially required for shared libraries; we shouldn't
just copy the artifacts built for platform, because they may be linking
against private (=unstable) symbols that are not available to APEXs
which are basically unbundled.

This CL, as a first step, makes cc.Module an APEX-aware module type.

Bug: 112672359
Test: m apex.test; the built apex has all the direct and transitive
shared lib dependencies of the libs and executables listed in Android.bp

Change-Id: I21f6a586654779984f0f5154b2a08b2adbf2168b