Show custom Attribution line instead of Author/Credit when available

  - Adds attribution variable to Image model
  - In mmv.ui.metadataPanel and mmv.EmbedFileFormatter, display
      attribution line instead of author and credit when it is set
  - Update junit tests for mmv.model.Image and mmv.EmbedFileFormatter

Bug: T67445
Change-Id: Idfe542a1542d28cf8d27c1720ab0bd54324b2f37
Merge "Show custom Attribution line instead of Author/Credit when available"