Move binderservicedomain policy to private

This leaves only the existence of binderservicedomain attribute as
public API. All other rules are implementation details of this
attribute's policy and are thus now private.

Test: No change to policy according to sesearch, except for
      disappearance of all allow rules to do with *_current targets
      referenced in binderservicedomain.te.
Bug: 31364497
Change-Id: Ic830bcc5ffb6d624e0b3aec831071061cccc513c