Return normalized file name on warning "exists-normalized"

Make API action=upload return normalized file name on warning
"exists-normalized" instead of filename to be uploaded to.

This enables humans to see which file the current upload conflicts
with.
There is no point in returning the filename the upload should go to
because the client that makes the request is usually aware of this.

Bug: 52699
Change-Id: I898b7f234d43eb1e524952f6de80e401a8c96671