android: camera_metadata: Add const version of get()

Add a const version of the CameraMetadata::get() method to retrieve
a const pointer to the camera metadata wrapped by the class instance.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

src/android/camera_metadata.cpp

@@ -50,3 +50,8 @@ camera_metadata_t *CameraMetadata::get()
 {
 	return valid_ ? metadata_ : nullptr;
 }
+
+const camera_metadata_t *CameraMetadata::get() const
+{
+	return valid_ ? metadata_ : nullptr;
+}