rust: Change default variants

rust_library now produces only rlib and dylib variants.
rust_library_foreign now produces static and shared variants.

This change was made because both are common cases, but all four
together essentially never happens. This allows us to have fewer repeat
module definitions.

Bug: 143217452
Test: cd external/rust; mma
Change-Id: Iaf69e8da38f7c12710331c51464699cf7cbbb656