rust: Only allow bindgen to produce `rlib`s.

Generated bindings are intended to be slim translation layers, usually
consisting of nothing more than type signatures and constants.
Generally, they should also be used in exactly one location by the safe
wrapper for these bindings. By preventing them from building as
`dylib`s, we avoid the per-library overhead of these non-reused pieces
of code.

Additionally, default visibility restrict all bindgen modules to their
subpackages. This is being done both:
* to encourage use of a single safe bindings crate
* to avoid diamond dependency graphs with mixed rlib/dylib dependencies

Bug: 166332519
Test: m; Make sample module use dylib bindgen dependency, see build failure.
Change-Id: I8e9d9cb851c2ec99f4ed63e6e18c4ba26b29721c