rust: Visibility exports for bindgen

We are now default restricting bindgen visibility to subpackages. This
change is part of a set of changes adding explicit visibility for
bindgen libraries which are already being used elsewhere.

* If these visibility rules are to allow access by the safe bindings
  library which lives elsewhere in the codebase, no action is needed.
* If safe bindings do not exist, and you own this library, now is a good
  time to think about producing them and transitioning your (now
  enumerated) client list.
* If safe bindings exist, but there is a surprise client of the raw
  bindings on this list, now is a good time to talk to them about why
  they don't want to use your safe bindings.

Bug: 166332519
Test: m
Change-Id: I1320a72d61cf0d760da3e037dec6ad35c8c979f8