Add __ANDROID_APEX__

A module can be built multiple times when it is referenced from one
or more APEXes. Sometimes, it is required for the module to behave
differently depending on the context; e.g., do A when built form
APEX M, do B when built for APEX N, and do C when built for platform.

The idea is to have a macro __ANDROID_APEX__ which is set to the
name of the apex that the module is built for. It is undefined when
the module is built for platform.

Bug: 122714993
Test: m (apex_test amended)
Change-Id: I11a89c6a0e799f4810194de7ef9ee472a71ca498