Add metadata helper class.

Breaks metadata down into distinct pieces (to be added later).

BUG: 29330282
TEST: unit tests pass

Change-Id: I7156e420e7cdb14b5c3155006ecefd8ff2a49500
diff --git a/modules/camera/3_4/v4l2_wrapper.h b/modules/camera/3_4/v4l2_wrapper.h
index fd4ac34..d67b36e 100644
--- a/modules/camera/3_4/v4l2_wrapper.h
+++ b/modules/camera/3_4/v4l2_wrapper.h
@@ -82,6 +82,8 @@
   // Lock protecting use of the device.
   std::mutex device_lock_;
 
+  friend class V4L2WrapperMock;
+
   DISALLOW_COPY_AND_ASSIGN(V4L2Wrapper);
 };