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/EmulatedVolume.h b/model/EmulatedVolume.h
index 131761c..4f76a60 100644
--- a/model/EmulatedVolume.h
+++ b/model/EmulatedVolume.h
@@ -46,6 +46,9 @@
     status_t doUnmount() override;
 
   private:
+    status_t mountFuseBindMounts();
+    status_t unmountFuseBindMounts();
+
     std::string getLabel();
     std::string mRawPath;
     std::string mLabel;
@@ -58,6 +61,9 @@
     /* Whether we mounted FUSE for this volume */
     bool mFuseMounted;
 
+    /* Whether to use sdcardfs for this volume */
+    bool mUseSdcardFs;
+
     DISALLOW_COPY_AND_ASSIGN(EmulatedVolume);
 };