Reset the progress to 0 when setting a non-action state

Per hughsie, when setting a non-action state, the apps' progress should
be reset to 0.

This patch adds that and also its coverage in the dummy plugin's tests.
1 job from wip/improve-tests in 9 minutes and 31 seconds (queued for 21 seconds)
Status Job ID Name Coverage
  Build
passed #32941
build-gnome-software

00:09:31