Add mode when open(O_CREAT) is used.

When creating a new file using open(..., O_CREAT), it is an error
to fail to specify a creation mode. If a mode is not specified, a
random stack provided value is used as the "mode".

This will become a compile error in a future Android change.

Change-Id: I73c1e1a39ca36bf01704b07302af4971d234b5a8