core: fix findbugs issue for PolicyUnit.equals()

PolicyUnitImpl is a decorator for PolicyUnit, but doesn't
overrides 'equals' method.
Fixed PolicyUnitImpl to only contain PolicyUnit.

Change-Id: I2d31dfb51268929df30bd468b14bfbc3e336c273
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>