audio: Refactor applet and extract singleton to MobileShellState

The eventual goal is to have as few singletons with state as possible in the mobileshell component when it is imported into components such as the lockscreen.

This doesn't fully accomplish it, but moves the audio provider singleton to MobileShellState, which will eventually need to be prevented from importing into non plasmashell processes.

This also disables the sound feedback when changing volume, since it can be a source of lag when showing the applet.