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.

(cherry picked from commit 2a4b5f9e)

Change-Id: Ie6e2027e59b49ca634c7308bcda7bb492d372f46