only apply volume changes iff it was set manually

otherwise defer to libvlc's default volume (75%).

when setting the volume unconditionally we may be overriding PA settings
(PASupport.isActive doesn't represent actual pa use since vlc 2.2), so
to prevent this from happening only set the volume iff the api user
explicitly requested the change. unitl such time applyVolume is basically
no-op

(specifically this prevent us from overriding the PA notification volume
by setting our explicit volume over the default volume. obviously this
also applies to every other restored sink-input though)