Concurrent export preparation is visually confusing #250

Procedure

1. Photograph a lot of pages (e.g. 20)
2. Go to the export screen with the lower right-hand button (on an LTR layout)
3. Wait a bit until the export progresses (e.g. to the half)
4. Go back to the photography screen (or to the settings, doesn't matter)
5. Go to the export screen again

Actual behavior

The export preparation appears to start anew, but the old one runs concurrently. The progress bar flickers back and forth between the two different progress states.

Once the old one finishes, the progress bar disappears, but the share/save buttons stay disabled. One needs to wait until the new one invisibly finishes, too; then the share/save buttons become available again.

Desired behavior

If going to the photography/settings screen did not make any changes; for the old export preparation to continue and not restart. If any changes are made, for the preparation to restart and the old one to be stopped for good.

(In the future, it might be even desirable to allow incremental export preparation while photographing? Not exactly the issue here though.)

Specs

Fairscan 2.3.0 installed via F-Droid on a Fairphone 3.

(Excellent app, by the way! Thanks for making it!)