init.rc: create /data/system/dropbox

Ensure dropbox has the new dropbox_data_file label.

Bug: 31681871
Test: ls -dZ /data/system/dropbox
    u:object_r:dropbox_data_file:s0 /data/system/dropbox

Change-Id: Ia67646f4a789155e20650c33fe4412cae7f930d2
diff --git a/rootdir/init.rc b/rootdir/init.rc
index 1462570..72f29fb 100644
--- a/rootdir/init.rc
+++ b/rootdir/init.rc
@@ -509,6 +509,7 @@
     mkdir /data/ss 0700 system system
 
     mkdir /data/system 0775 system system
+    mkdir /data/system/dropbox 0700 system system
     mkdir /data/system/heapdump 0700 system system
     mkdir /data/system/users 0775 system system