Sync on Android reuploads files to other nodes, resetting their mtimes

[himselfv]

Windows PC: Sync 2.7.3 (1381)
Android: Sync 2.6.5 (9641)

How to reproduce:

1. Create an encryption-supporting share on Windows PC from a folder with a bunch of 2-3Mb sized JPEGs (120Mb is enough). Make sure the files' modification times are not recent.

2. Add a full node on Android. Mark it for full sync (no selective sync). Wait for full sync. At this point everything is usually fine.

3. Quit Sync on Android. Wait for some time. Usually a few hours is enough, maybe up to a day. Restart Sync on Android and wait for it to index things and work for a while.

4. Observe: modification times of some files on PC are now recent (the times when these files had initially been synced to Android).

I've read elsewhere that many Android builds like to mess with modtimes and so Sync ignores file system modtimes on Android and only cares about the ones it recorded in its DB, but here it feels like it doesn't do that, maybe?

Notes:

* Instead of the full sync you can usually do selective sync and just select a few files, and only those will be affected.

* I don't know if it's important that the share is encryption-supporting. Haven't tried this on basic shares.

* I've studied the logs on the PC side, and for all affected files, the PC discovers there are "newer versions" (higher mtime) at the Android node, downloads those and that's why local files on the PC change modtime. (The files aren't really being changed anywhere!)

* There's not way to read the logs on the Android, so can't tell what happens there.

* Android filesystem modtimes for those files are indeed recent - either Sync doesn't even bother to set them on Android, or it's one of those systems where they don't stick.

* Each time you run this experiment, only some files get broken, not all of them. If you run Sync again and again for several days, more of the files are becoming affected, but some still retain their original modtimes on the PC.

* I think I had the same thing happen to me when the android node is read-only, but that would be extremely weird and I can't reproduce it now, so maybe I'm mistaken.