Improve GUI handling of zip flashing

- change the page sooner so that users cannot activate the slider
  twice
- improve GUI messaging if a zip requests a cache wipe
- move update of the file count to the end so it is less visible
  to the user (e.g. during a cache wipe the counter should not
  read 0)

Change-Id: I5d478f07effe61ca37f3521a77c9e9243a9cb692