Conditionally use sdcardfs.

In preparation of sdcardfs going away on devices launching with R,
conditionally use it.

Bug: 146419093
Test: cuttlefish with sdcardfs, cuttlefish without sdcardfs but with
FUSE
Change-Id: I2c1d4b428dcb43c3fd274dde84d5088984161993
diff --git a/model/PublicVolume.h b/model/PublicVolume.h
index dd76373..3156b53 100644
--- a/model/PublicVolume.h
+++ b/model/PublicVolume.h
@@ -68,6 +68,9 @@
     /* Whether we mounted FUSE for this volume */
     bool mFuseMounted;
 
+    /* Whether to use sdcardfs for this volume */
+    bool mUseSdcardFs;
+
     /* Filesystem type */
     std::string mFsType;
     /* Filesystem UUID */