A previous security fix meant we no longer stored apk files on the
SD card. However, this should still be a feature that people can opt
for if they want, without being insecure. As such the process is now:
* First download: put in internal storage (to ensure it can't be
modified before installing)
* After download: also copy to SD card for caching.
* On starting F-Droid:
+ Always delete internal storage apks.
+ Only delete other, cached apks if cache preference is false.
To make the code simpler and less prone to bugs, I had to consider
the fact that if people did not have an accessible SD card, then the
path to a cached apk and a "downloaded but transient" apk cannot be
the same. While possible, it means many checks to see if they are
the same, thorough permission management to prevent security issues,
and makes it harder to clear transient apks when F-Droid starts.
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