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/libs/apkverify/Android.bp b/libs/apkverify/Android.bp
index 2d58bb0..ab9265d 100644
--- a/libs/apkverify/Android.bp
+++ b/libs/apkverify/Android.bp
@@ -7,7 +7,7 @@
     crate_name: "apkverify",
     srcs: ["src/lib.rs"],
     prefer_rlib: true,
-    edition: "2018",
+    edition: "2021",
     rustlibs: [
         "libanyhow",
         "libbyteorder",
@@ -37,7 +37,7 @@
     crate_name: "apkverify_test",
     srcs: ["tests/*_test.rs"],
     prefer_rlib: true,
-    edition: "2018",
+    edition: "2021",
     test_suites: ["general-tests"],
     rustlibs: [
         "libapkverify",