Allow vold to create subdirectories within its private area.

AVC message we get without this change creating
/data/misc/vold/user_keys

05-12 08:57:54.839   189   189 W vold    : type=1400 audit(0.0:176):
avc: denied { create } for name="user_keys" scontext=u:r:vold:s0
tcontext=u:object_r:vold_data_file:s0 tclass=dir permissive=0

Bug: 19704432
Change-Id: I10fc90c64c7c10fa9811133a4289663ae8617607