Camera: fix metadata assertion

Bug: 19897963
Change-Id: I6b383c4750f31691a1c02e927bbeb0c1998a9eff
diff --git a/camera/CameraMetadata.cpp b/camera/CameraMetadata.cpp
index 043437f..e216d26 100644
--- a/camera/CameraMetadata.cpp
+++ b/camera/CameraMetadata.cpp
@@ -74,7 +74,7 @@
     clear();
 }
 
-const camera_metadata_t* CameraMetadata::getAndLock() {
+const camera_metadata_t* CameraMetadata::getAndLock() const {
     mLocked = true;
     return mBuffer;
 }