Bug 870608 - Restoring volume on phone start

Fix for [#870608](https://bugzilla.mozilla.org/show_bug.cgi?id=870608). When we currently start up the phone we add a bunch of observers for the settings related to the volume. All good. However, we skip the check if we have any pending requests. When the first result comes in, it can [trigger a chain of reactions](https://github.com/comoyo/gaia/blob/123dd1dc3dcc102f94d58f74c05a4745618ef022/apps/system/js/sound_manager.js#L293) which then spawns new pending requests, causing the other loaded settings to be omitted and the volume of f.e. the ringer to be kept at the original of 15. This patch makes sure that the initial load event for every setting will bubble through.