Use lastUpdateMillis of an apex for ART versioning.

Test: m
Bug: 192647837
Change-Id: Ide7e5a1dbfe85c679313bb6c8e6788137b138f42
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index 5a49f50..d588ff7 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -1309,7 +1309,8 @@
       if (info == apex_infos.end() || info->second->getIsFactory()) {
         result += '/';
       } else {
-        android::base::StringAppendF(&result, "/%" PRIu64, info->second->getVersionCode());
+        // We use the mtime provided in the format as a version number.
+        android::base::StringAppendF(&result, "/%" PRIu64, info->second->getLastUpdateMillis());
       }
     }
 #endif