Fixes#9564
Avoids the issue of clearing the cache when the app is backgrounded despite being in the reader.
We could do a job on idle, but we'd still need to be careful around whether the reader is active,
so this is just simpler considering it's a separate activity.
(cherry picked from commit 53c6230afebe8e8cddca216f281fdec70e450a33)
# Conflicts:
# app/src/main/java/eu/kanade/tachiyomi/ui/main/MainActivity.kt
e26360186a