Change header message when saving in VE

Change from "Previewing [page]" to "Saving [page]" since we're not
previewing anything. Update an existing unused message and use it (this
is probably temporary since VE edit summary will be in a flyout in
future).

Bug: 61050
Change-Id: Id8ef3672b5037f3da805ceb7c73d2506392f23d9
Merge "Change header message when saving in VE"