Fix ring/notification volume not restoring correctly on headset (dis)connect

We weren't checking the alias of each stream when restoring, which meant that
for the notification stream volume:
1. The wrong volume level would be stored.
2. The volume level to restore would be retrieved from the wrong place, and then
overwrite the correctly restored ring stream volume.

As well as fixing that I have improved the readability of the code a bit.

Change-Id: I95234c2b7ad7dfd4dbd455a56c8a69975c514b50