protected-apps: don't show a progress dialog for short operations

This code has been ported from a time where devices were much lower
end and currently the operation of writing protected apps state to
permanent storage is really fast. However, the progress dialog would
still flash for a few milliseconds but it is in fact useless since the
user cannot read it and will only see it flash.

This patch delays showing the dialog for 50ms which will ensure that
modern devices won't actually show it but slower devices will still
see it.

Change-Id: I43c347822d205bd930912ea5c03951e2abb9354d