Remove explict use of ASCII encoding

Replace the use of CFStringGetCString with kCFStringEncodingASCII specified
with CFStringGetFileSystemRepresentation which will ensure the correct
character encoding is used to conver the CFString into a NULL terminated
char array suitable for use with POSIX APIs.

Change-Id: Ibab1dc05c4f4db8604d329a493b4241992b8e69d