StubVolume as first class Volume in Vold

StubVolume is a Volume that is maintained by external party such as the
ChromeOS processes in ARC++.

Bug: 110380403
Test: Tested on ARC++

Change-Id: I3198bd7283d5f60a524da3707dea7385ffec599d
diff --git a/model/VolumeBase.h b/model/VolumeBase.h
index 6532a80..92a83f0 100644
--- a/model/VolumeBase.h
+++ b/model/VolumeBase.h
@@ -54,6 +54,7 @@
         kEmulated,
         kAsec,
         kObb,
+        kStub,
     };
 
     enum MountFlags {