Use "auto" for microdroid super image size

Bug: 181107248
Test: boot microdroid
Change-Id: I2f4787e9d53429e7b3ec3f5b6adb20ab0205bf28
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index adb9ac8..82be5ae 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -119,8 +119,7 @@
 logical_partition {
     name: "microdroid_super",
     sparse: true,
-    // TODO(b/181107248): remove this hard-coded value
-    size: "104857600",
+    size: "auto",
     default_group: [
         {
             name: "system",