apex: __ANDROID_APEX__ defined with no value

__ANDROID_APEX__ was defined with the name of apex module.
-D__ANDROID_APEX__=com.android.foo

But in this way, conditional compilation is not easy since comparing
macro's string value is not supported in C/C++.
(There's no usages of this value in source tree.)

In most cases, modules can check if __ANDROID_APEX__ is defined to see
if they are compiled for apex.
For modules which should behave differently according to which apex they
are included, they can check __ANDROID_APEX_<NAME>__.

Bug: 142582178
Test: m (soong tests run) && boot device && TH
Change-Id: I0f5e3e9463ccd96cbba333a8bdd648470c5c912d