Add initial sepolicy for app data snapshots.

Define a rollback_data_file label and apply it to the snapshots
directory. This change contains just enough detail to allow
vold_prepare_subdirs to prepare these directories correctly.

A follow up change will flesh out the access policy on these
directories in more detail.

Test: make, manual
Bug: 112431924

Change-Id: I4fa7187d9558697016af4918df6e34aac1957176