Migrate to Rust 2021 edition

I only wanted this in one place, but thought we might as well do it
all in one go.

In a couple of places I added an explicit edition instead of using the
default (whatever that is).

See https://doc.rust-lang.org/edition-guide/rust-2021/index.html.

Test: find . -name "Android.bp" -type f|xargs grep edition | grep -v 2021
Test: atest ComposHostTestCases MicrodroidHostTestCases
Change-Id: I4de050078b303cc5614d230e0d36d5454d60a807
diff --git a/authfs/Android.bp b/authfs/Android.bp
index 4c5b70e..7788702 100644
--- a/authfs/Android.bp
+++ b/authfs/Android.bp
@@ -8,7 +8,7 @@
     srcs: [
         "src/main.rs",
     ],
-    edition: "2018",
+    edition: "2021",
     rustlibs: [
         "authfs_aidl_interface-rust",
         "libandroid_logger",