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/Android.bp b/Android.bp
index 8f2ba34..44e2317 100644
--- a/Android.bp
+++ b/Android.bp
@@ -129,6 +129,7 @@
         "model/PrivateVolume.cpp",
         "model/PublicVolume.cpp",
         "model/VolumeBase.cpp",
+        "model/StubVolume.cpp",
         "secontext.cpp",
     ],
     product_variables: {