service/am: Unstub SetExpectedMasterVolume()

This function passes in the desired main applet and library applet
volume levels. We can then just pass those values back within the
relevant volume getter functions, allowing us to unstub those as well.

The initial values for the library and main applet volumes differ. The
main applet volume is 0.25 by default, while the library applet volume
is initialized to 1.0 by default in the services themselves.