Allow apps to execute app_data_files

Fixes the following denial:

<5>[28362.335293] type=1400 audit(1378991198.292:24): avc:  denied  { execute } for  pid=1640 comm="facebook.katana" path="/data/data/com.facebook.katana/app_libs/libfb_jpegturbo.so" dev="mmcblk0p23" ino=652556 scontext=u:r:untrusted_app:s0 tcontext=u:object_r:app_data_file:s0 tclass=file

Change-Id: I4a515610149f06f0c49194feb6bc96e9b3080c12