Use java.nio.file consistently in FS

Use java.nio.file consistently in FS

Since 4.0 we require Java 7 so there is no longer a need to override the
following methods in FS_POSIX, FS_Win32, FS_Win32_Cygwin
- lastModified()
- setLastModified()
- length()
- isSymlink()
- exists()
- isDirectory()
- isFile()
- isHidden()
Hence implement these methods in FS and remove overrides in subclasses.

Change-Id: I5dbde6ec806c66c86ac542978918361461021294
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>