Define __ANDROID_VENDOR_API__ for product variants

As the product variants are also able to use LLNDK libraries, they
must know the version of the LLNDK libraries that the system is
building.
Provide the __ANDROID_VENDOR_API__ for both product and vendor
variants for this purpose.

Bug: 368559337
Test: TH
Change-Id: If4e80006641b42b3b08e0b48a49d319ae05e567e