instance.img is actually added as a writable partition

Bootloader doesn't use the partition yet.

Bug: 193504400
Test: atest MicrodroidHostTestCases
Change-Id: I7138d966630ebf79c9b7cbc2ea7343852460e2ec
diff --git a/microdroid/microdroid.json b/microdroid/microdroid.json
index b1ef86a..4562f15 100644
--- a/microdroid/microdroid.json
+++ b/microdroid/microdroid.json
@@ -30,10 +30,11 @@
       "partitions": [
         {
           "label": "uboot_env",
-          "paths": ["/apex/com.android.virt/etc/uboot_env.img"]
+          "paths": ["/apex/com.android.virt/etc/uboot_env.img"],
+          "writable": false
         }
       ],
-      "writable": false
+      "writable": true
     }
   ]
 }