When a stub is built for APEX, it is generated with --apex

Now, symbols that are only to be visible to across APEXes can be tagged
as # vndk. Then when generating the stubs library, the tagged symbol
is included. The symbol is NOT included in other cases; build NDK stubs,
etc.

Bug: 120638081
Test: m (apex_test updated.)
Change-Id: Idb2b552badddfc26af113cc8d4b984788f478813