Use the vm tool

MicrodroidHostTestCases now uses the vm tool to create the VM. It no
longer directly interacts with crosvm and mk_cdisk.

The READMD.md file is rewritten to fully reflect the recent changes, and
also to add description about building an app for microdroid.

Bug: 185891097
Test: atest MicrodroidHostTestCases
Change-Id: I5fdc854390fd362ebead22a4a36af75c30500a81
diff --git a/apex/Android.bp b/apex/Android.bp
index 146e4eb..bf38860 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -54,8 +54,7 @@
     ],
     prebuilts: [
         "com.android.virt.init.rc",
-        "microdroid_cdisk.json",
-        "microdroid_cdisk_env.json",
+        "microdroid.json",
         "microdroid_uboot_env",
         "microdroid_bootloader",
     ],