Don't use qcow2 for userdata.

We provide a sparse file directly as a (writable) partition image and
format a filesystem on the partition inside the VM.

To format it only when it's not formatted, the option `formattable` is
added to the fstab entry for the userdata partition.

Bug: 185767624
Test: atest MicrodroidHostTestCases
Change-Id: Ia938fd60071e64170b8280d19c17c4413c769337
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index 435557e..3ace4a6 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -56,6 +56,8 @@
         "logd",
         "run-as",
         "secilc",
+        "mke2fs",
+        "e2fsdroid",
 
         // "com.android.adbd" requires these,
         "libadbd_auth",