vold: const-ify some of the API

We make some of the C++ member functions 'const', to reflect how
they (don't) affect the state of the class.

Test: TreeHugger
Change-Id: Iec1c2801bfe721e2741406ed1ac0ef95662840a6
diff --git a/model/PrivateVolume.h b/model/PrivateVolume.h
index 85aa4dc..cb8e75d 100644
--- a/model/PrivateVolume.h
+++ b/model/PrivateVolume.h
@@ -39,9 +39,9 @@
   public:
     PrivateVolume(dev_t device, const std::string& keyRaw);
     virtual ~PrivateVolume();
-    const std::string& getFsType() { return mFsType; };
-    const std::string& getRawDevPath() { return mRawDevPath; };
-    const std::string& getRawDmDevPath() { return mDmDevPath; };
+    const std::string& getFsType() const { return mFsType; };
+    const std::string& getRawDevPath() const { return mRawDevPath; };
+    const std::string& getRawDmDevPath() const { return mDmDevPath; };
 
   protected:
     status_t doCreate() override;