SELinux policies for /data/preloads directory

A new directory is created in user data partition that contains preloaded
content such as a retail mode demo video and pre-loaded APKs.

The new directory is writable/deletable by system server. It can only be
readable (including directory list) by privileged or platform apps

Bug: 28855287
Change-Id: I3816cd3a1ed5b9a030965698a66265057214f037