fix(plugins): trigger plugin hooks when saving plugin settings

As stated in #6820 when the change to setPrivateSetting was introduced it doesn't have in account that calling directly that method doesn't triger the plugin hooks.
In the comment from <a href="https://github.com/Elgg/Elgg/issues/6820#issuecomment-42271849">@jeabakker</a> he explains why ``->`` should be called instead of ``->set`` (deprecated) or ``->setSetting()`` (don't call the plugin hooks).

Fixes #6820