Support building all variants.

Bug: 143217452
Test: cd external/rust; mma; atest in all TEST_MAPPING subdirs
Change-Id: I39a1b17c126912eed7d2e6f015e72a37bc0c663a
diff --git a/Android.bp b/Android.bp
index eb6a4c4..d199fe7 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_executor",
     host_supported: true,
     crate_name: "futures_executor",
@@ -10,7 +10,7 @@
         "default",
         "std",
     ],
-    rlibs: [
+    rustlibs: [
         "libfutures_core",
         "libfutures_task",
         "libfutures_util",