Emit LOCAL_CHECK_SAME_VNDK_VARIANTS

If a VNDK library does not require using different core and vendor
variants, emit LOCAL_CHECK_SAME_VNDK_VARIANTS so that the two variants
can be checked for identicalness.

Bug: 145157349
Test: With the corresponding change in build/make, remove libbinder
      from build/soong/cc/config/vndk.go and check a build fails
      even without TARGET_VNDK_USE_CORE_VARIANT set.

Change-Id: I7698edf9a24b0df150c0e4f7d8dd4926f053eee8