AI 144950: Manual integration of 144342 and 144547 from donutburger to cupcake.

  Fix bag (string-array, etc) behavior with overlays.
  We used to replace elements in the default with elements from the
  overlay.  This change causes us to empty the array first so if the
  overlay array is smaller we don't end up with elements from the
  default array showing through at the end of the final result.
  Ex: [A,B,C] default with [D] overlay should give [D] but used to
  give [D,B,C]
  BUG=1754390

Automated import of CL 144950