rust: Enable Rust modules in Product

The prohibition on Rust dylibs outside system still stands, but rlibs,
rust_ffi_static, and binaries will all work fine.

Test: m nothing (new soong tests added by this commit)
Test: Created sample product_specific rust_binary, checked out/ location
Bug: 165791368
Change-Id: I6453274064bb24b2019f38e57fc0d09b7c0fcf30