ANDROID: Re-enable Rust symbol export

Previously, we disabled Rust symbol export because we were planning on
testing Rust Binder as a builtin component.

Rust symbols should still not be allowed on the stable KMI. This exports
Rust runtime symbols with the fake type `int`, which means that
MODVERSIONS will always be permissive.

This change is Android-only because we are pursuing a more complete
strategy upstream, but this is suitable for a GKI-module trial of the
Rust binder driver:

https://lore.kernel.org/rust-for-linux/20240617175818.58219-17-samitolvanen@google.com/T/#t



Bug: 347787665
Change-Id: I35dd3b532790f292f19b27670d1f0271be937878
Signed-off-by: Matthew Maurer <mmaurer@google.com>