File deletion improvement

[Doplgangr]

The original classes are quite messy. After some thoughts and experimentation here are some improvements

• Silencing deleting functions so that they do not repeatedly result in a rerun, and causing unnecessary us of computational power.
• Reorder functions so that there would always be a final deletion function. (In some samsung devices, the shredding function creates a file with 0 filesize.)
• reducing size of log so that the >10 reruns do not create a massive congestion in the logcats.
• More sensible comments.

[Doplgangr]

Not sure about that, but I tried it on an actual device (Nexus 4) and it worked. File was originally discoverable by Filedigger and with subsequent deletion it was no longer discovered.

Have recieved one of the user who deleted his files and regretted, also reported that file diggers reports nothing.

It is possible that flash memories will be worn out if such actions are done in a prolonged manner, but does it outweight the possible security improvement?

[L-Henke]

Filedigger might report nothing because it couldn't read the complete files, but that doesn't mean that everything of that file has been deleted. That is almost impossible on current flash memories.

Of course your method might overwrite parts of the original file, but we should not sell the users that their files are securely deleted, because that simply is not true.

If your are interested in the details, here is a scientific paper about secure file deletion from flash memory: https://www.usenix.org/legacy/events/fast11/tech/full_papers/Wei.pdf

Quote from the conclusion:

We found that none of the available software techniques for sanitizing individual files were effective.

[Graveen]

This PR worth merging, for the 0 byte ghost file remaining.