Move rules / neverallow assertions from public to private policy. This
change, by itself, is a no-op, but will make future patches easier to
read. The only downside of this change is that it will make git blame
less effective.

Motivation: When rules are placed into the public directory, they cannot
reference a private type. A future change will modify these rules to
reference a private type.

Test: compiles
Bug: 112357170
Change-Id: I56003409b3a23370ddab31ec01d69ff45c80d7e5