Allow "mkdir /sdcard/foo"

Creation of directories on /sdcard was not allowed for the
shell user. Fixed.

Steps to reproduce:

adb shell
cd /sdcard
mkdir foo

Addresses the following denial:

<5>[  446.361215] type=1400 audit(13427374.189:13): avc:  denied  { create } for  pid=3264 comm="mkdir" name="foo" scontext=u:r:shell:s0 tcontext=u:object_r:sdcard_internal:s0 tclass=dir

Bug: 12778268
Change-Id: I5a91b7581fe7e97fbf3caeed9fdf74981e49c58e

shelldomain.te

@@ -6,7 +6,7 @@ allow shelldomain shell_data_file:file create_file_perms;
 allow shelldomain shell_data_file:file rx_file_perms;
 
 # Access sdcard.
-allow shelldomain sdcard_type:dir rw_dir_perms;
+allow shelldomain sdcard_type:dir create_dir_perms;
 allow shelldomain sdcard_type:file create_file_perms;
 
 # adb bugreport