Clean up AtomicFile and make it return a custom FileOutputStream
for writing which handles IOException automatically during write
operations.
It also syncs the file descriptor and deletes the backup file on
close() call. This fixes the order of flush and close operations
when the fileoutputstream is wrapped by another OutputStream.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=138779187
