Emulated volumes above private volumes.

When a private volume is mounted, create an emulated volume above it
hosted at the /media path on that device.  That emulated volume is
automatically torn down when unmounting the private volume.

Add "removed" state for volume, which signals to framework that
media has left the building, send when the volume is destroyed.

Bug: 19993667
Change-Id: I1f82b51de578ac5cfcc5d7b9a6fb44f6f25c775c
diff --git a/VolumeBase.h b/VolumeBase.h
index 1494c2c..e7c7836 100644
--- a/VolumeBase.h
+++ b/VolumeBase.h
@@ -75,6 +75,8 @@
         kUnmounting,
         /* Next states: mounting, formatting */
         kUnmountable,
+        /* Next states: none */
+        kRemoved,
     };
 
     const std::string& getId() { return mId; }