The UI shows the "Unmark Private" button for merged changes, but
then the operation fails because SetPrivateOp has a check for the
change's status and rejects the operation when it's merged.
Remove the check from SetPrivateOp and modify the existing checks
in PostPrivate and DeletePrivate to allow administrators to perform
the operation on merged changes.
Split DeletePrivateByPost out to a separate class rather than being
a static subclass of DeletePrivate. This allows to reuse the utility
method canDeletePrivate, which was not possible from a static class.
Bug: Issue 6211
Change-Id: I507f2d8b09dfe0a2726e0a7bede26a5be8cc7fc1
d0441d1a71