Privacy Panel new pull request

This is a new pull request. We had to remove the old one due to big problems with git and some Schroedinger patches....
Finally, we made `appsCache` before this patch got landed. Because we all know that `mozApps.mgmt.getAll()` would hurt performance a lot (with some regression bugs), in order to fix this, `appsCache` was made to fix this.

please follow https://github.com/mozilla-b2g/gaia/blob/bf1a629614c9454117e96002284cd0878b4c94ac/apps/settings/js/panels/root/themes_item.js#L60 to get apps !