Set base_dir of microdroid to system

Bug: 179652970
Test: see contents of microdroid.img
Change-Id: I67abae5e308ea1f0d67b426fc2bcaa36773634cc
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index f163d77..012df71 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -10,6 +10,7 @@
     deps: [
         "init_second_stage",
     ],
+    base_dir: "system",
     file_contexts: "microdroid_file_contexts",
 }