Allow overriding PRODUCT_EXTRA_VNDK_VERSIONS

As a PRODUCT variable, PRODUCT_EXTRA_VNDK_VERSIONS is not allowed to
be assigned from the build command. Instead, we can use
OVERRIDE_PRODUCT_EXTRA_VNDK_VERSIONS in the build command to
override PRODUCT_EXTRA_VNDK_VERSIONS.

Bug: 132412510
Test: OVERRIDE_PRODUCT_EXTRA_VNDK_VERSIONS=28 m vndk_snapshot_package
Change-Id: I91579961ab393f40ddce8bfe9304829b5b669441