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/compos/Android.bp b/compos/Android.bp
index ec2d737..6aa9d3d 100644
--- a/compos/Android.bp
+++ b/compos/Android.bp
@@ -4,6 +4,7 @@
 
 rust_defaults {
     name: "compsvc_defaults",
+    edition: "2021",
     srcs: ["src/compsvc_main.rs"],
     rustlibs: [
         "android.system.virtualmachineservice-rust",