Fix 'original dimensions' async call in MediaEdit dialog

The request for originalDimensions is taken from the API, which can be
rather slow. There is a future (soon) fix that refactors the entire way
we read the originalDimensions asynchronously and load it into the dialog
but until that is available, this fix introduces a couple of basic
fallbacks in case originalDimensions are not yet available in the size
widget.

Bug: 62024
Change-Id: I8d00cea6f1d667359a44a6c185c16340bc6e81c9