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....
Let's narrow down the scope and focus on testing `_getApp` method instead, because it is a promise, please follow https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/test/unit/modules/apps_cache_test.js#L62 to write tests.