AI 144342: Fix bag (string-array,etc) behavior with overlays.

AI 144342: 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 array.
  Ex: [A,B,C] and overlay [D] should give [D] but used to give
  [D,B,C].
  BUG=1754390

Automated import of CL 144342