rust: Emit android vndk cfg flag.

Pass an "android_vndk" cfg flag that indicates this code
targets the vndk. This can be useful in instances where code
might need to behave differently.

This also includes a fix to make sure our vendor Soong tests
are correctly configured.

Bug: 179907868
Test: Soong tests pass.
Test: Example module emits new cfg flags.
Change-Id: I01cdf91f6f9d42cd8a759266d5170479664bf4bc