rust: Add rust_ffi_static vendor ramdisk Support

Similar to our vendor support, this adds support for linking rust static
libraries to vendor ramdisk cc modules.

A bug fix is also included where a restriction against setting rust_ffi
vendor-specific was not being enforced.

Bug: 179397942
Test: Example modules link, Soong tests pass.
Change-Id: I737cdf0c2f49ab349bcea2a0429e6298ebc1313e