android: camera_metadata: Mark isValid() as const

The CameraMetadata::isValid() method does not modify the object state
and can be called on const instances of the CameraMetadata class.

Mark the method as const to allow that.

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.h

@@ -17,7 +17,7 @@ public:
 	CameraMetadata(size_t entryCapacity, size_t dataCapacity);
 	~CameraMetadata();
 
-	bool isValid() { return valid_; }
+	bool isValid() const { return valid_; }
 	bool addEntry(uint32_t tag, const void *data, size_t data_count);
 
 	camera_metadata_t *get();