Support building all variants.

Bug: 143217452
Test: cd external/rust; mma; atest in all TEST_MAPPING subdirs
Change-Id: I63a25f46eeba5a31c8bfa5b56b6c7ab85d66a7c7
diff --git a/Android.bp b/Android.bp
index 449aa67..7923625 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1,6 +1,6 @@
 // This file is generated by cargo2android.py --run --device.
 
-rust_library_rlib {
+rust_library {
     name: "libfutures_core",
     host_supported: true,
     crate_name: "futures_core",