C++ version of vold_create_subdirs needs extra permission

10-23 16:40:43.763  7991  7991 I auditd  : type=1400 audit(0.0:79): avc: denied { open } for comm="vold_prepare_su" path="/dev/pts/1" dev="devpts" ino=4 scontext=u:r:vold_prepare_subdirs:s0 tcontext=u:object_r:devpts:s0 tclass=chr_file permissive=0

Bug: 67901036
Test: Boot device, create user, create files, remove user, observe logs

Change-Id: I8d33dfd2a0b24611773001f20101db40aeb13632