Use mk_cdisk instead of assemble_cvd

mk_cdisk is a tool to create a composite disk image. It can be used to
create other kinds of composite disks as well as os_composite.os which
was created by assemble_cvd before.

For example, if we want to pass host apexes to microdroid we can pass
them via a composite disk referencing host apex files.

Bug: 181093750
Test: running microdroid VM following updated microdroid/README.md
TEst: MicrodroidTestCase
Change-Id: Ief6468564da9eb78c6729ae64a218811919b3ae0
diff --git a/apex/Android.bp b/apex/Android.bp
index bb73630..5357566 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -29,7 +29,7 @@
         },
     },
     binaries: [
-        "assemble_cvd",
+        "mk_cdisk",
         "fd_server",
         "virtmanager",
         "vm",