selinux: add domain for snap

Snap needs to read vendor files on treble enabled devices.

This is based on policies written by Eino-Ville Talvala <etalvala@google.com> for Google camera
app.

Change-Id: Ia4f3da3fa20ff894b5b202f6691e8f2c03f06e7f
(cherry picked from commit e61f6cdc)