Check file existing before set write permission on dso file
Check file existing before set write permission on dso file

Summary: `java.io.File.setWritable` will return false if the file doesn't exist. To avoid to print too much false warning on fresh installation. We need check the file existing before the printing warn log.

Differential Revision: D25537519

fbshipit-source-id: 7d0740dfe6e2d3e19189c5573c418781d6e3c5fe

facebook/SoLoader,facebook/SoLoader,facebook/SoLoader