Bugfixes and improvements for the progress bar

- the bar now starts at 5% for a visual indication that
something is going on
- the bar animates (fast) to 100%, instead of
disappearing immediately
- the animation logic has been fixed to avoid seeing
the bar go backwards
- added sanity check in all the callbacks to make sure
that we don't apply any changes to an image we are
not looking at anymore (including progress updates)

Bug: 58055
Change-Id: I765a61c16513e9330a412c5ec96387623ae7dbc7
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/146
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/242
Merge "Bugfixes and improvements for the progress bar"