Check for product variant in addition to core variant.

It's possible that a module will have a product variant
but not a core variant. In that case, we still need to
apply the vendor suffix to the androidmk module name.

Bug: 188720375
Bug: 187963715
Test: Build vendor image using vendor snapshot.

Change-Id: I190fdd707ca23357d1c324c1b1f3318457625f03