image cache: mark as android cache

Android 4.4 here with the app v2.0.1 (105) from f-droid. I tried the image cache, but then realized it uses too much SD card space (due to #601 ). I disabled the option, which did not delete the cache (ok for me) but looking at Android settings > Apps > wallabag I could see that the images are accounted as app data instead of app cache. I'd like to delete the images by clearing the apps' cache, but thus I cannot. clearing the data would also delete the apps' login credentials, which I think is problematic.
So please mark the image cache as app cached data.

No file or test is named. Locate the image-cache implementation and its Android storage handling, then verify that cached images are treated as app cache, can be removed by clearing the app cache, and remain separate from login data.

Written by the indexing model from the issue text.