Implement rust_aconfig_library

rust_aconfig_library generates src/lib.rs and uses it to build library variants (dylib, rlib-rlib_std, and rlib-dylib_std) as of what `rust_library` produces

Test: go test
Change-Id: I6c4603691d4306c463c2e9521f5c11c30765b1e3
diff --git a/aconfig/Android.bp b/aconfig/Android.bp
index 6927765..d2ddfdf 100644
--- a/aconfig/Android.bp
+++ b/aconfig/Android.bp
@@ -14,6 +14,7 @@
         "soong-bazel",
         "soong-android",
         "soong-java",
+        "soong-rust",
     ],
     srcs: [
         "aconfig_declarations.go",
@@ -24,6 +25,7 @@
         "init.go",
         "java_aconfig_library.go",
         "testing.go",
+        "rust_aconfig_library.go",
     ],
     testSrcs: [
         "aconfig_declarations_test.go",