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/microdroid/Android.bp b/microdroid/Android.bp
index 8ffb3cb..90d2678 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -298,3 +298,8 @@
         "microdroid",
     ],
 }
+
+prebuilt_etc {
+    name: "microdroid_cdisk.json",
+    src: "microdroid_cdisk.json",
+}