Add __ANDROID_APEX_<NAME>__ for apex variants

With __ANDROID_APEX_<NAME>__ definition, native modules may have
different behavior when it is built for a specific apex module.

Previously, the name is passed as value of definition __ANDROID_APEX__
like -D__ANDROID_APEX__=com.android.foo. But it is difficult to do
conditional compilation with it.

Now, since the name is incorporated into definition itself, it gets
easier to set #ifdef condition.

Bug: 142582178
Test: m (soong test added)
Change-Id: I3c90c789fa692a19addf2e5a7c8d4cc571cde112