Reduce Microdroid's default memory

Microdroid should not have such a large default memory size. The
original value was once required by CompOS (which now sets its own
value since aosp/1957659).

Bug: 215768835
Test: atest MicrodroidTestCase AuthFsHostTest
Change-Id: I18061807dd92799e5f369ecb0aa5807b0de3245c
diff --git a/microdroid/microdroid.json b/microdroid/microdroid.json
index c6c743d..0c294e9 100644
--- a/microdroid/microdroid.json
+++ b/microdroid/microdroid.json
@@ -37,6 +37,6 @@
       "writable": true
     }
   ],
-  "memory_mib": 2048,
+  "memory_mib": 256,
   "protected": false
 }