aidl_interface for cpp must not create product variant

VNDK variant of libbinder is available only to vendor modules.
Instead of setting the 'product_available: false' that may create
product variant, keep the 'product_available' property nil.

Bug: 150902910
Test: build
Change-Id: Ic0aa3295cfe201c35105b21a2d0a664eccffacdb
1 file changed