[Tizen]  Correct namespace for file operation functions inside package_installer.cc

[Tizen]  Correct namespace for file operation functions inside package_installer.cc

After upgrade Chromium version to 31.0.1650.12, some file operation functions are
moved from namespace 'file_util' to 'base', including PathExists(),
CopyFile(), DeleteFile() .etc.

This CL is to correct namespace for these functions inside
package_installer.cc.