Handle the case when a symbol is annotated with "# apex vndk"

This change fixes a bug that a symbol is omitted for apex (or vndk) when
it is annotated with "# vndk apex" (or "# apex vndk).

Bug: 123349183
Test: python3 test_gen_stub_libs.py

Change-Id: I344d6e70732bae8877cb16bbe881edb79fe90670