Ensure that the thumbnail path is valid even without a thumbnail.

- When there is no thumbnail snapshot, an empty sized thumbnail path is
  created since thumbnailWidth/Height are zero.  Instead, like the parent
  class, the path should default to the view width/height in such a case.

Bug: 66938643
Test: Launch PlayMovies video anew, go home, then to overview
Change-Id: I354ff0716ee44149f50ff11fa435161cf466c0cc