AudioService: remove DEVICE_OUT_SPDIF from mFixedVolumeDevices

For ATV box product, the SPDIF port should be available for audio output.
But SPDIF doesn't support hotplug as HDMI, we have to leave it as an attached device inside audio_policy_configuration.
This means SPDIF is always considered as connected.
<attachedDevices>
    <item>Spdif</item>
    <item>Built-In LoopBack</item>
</attachedDevices>

And that AudioService treats DEVICE_OUT_SPDIF as fixed volume devices.
int mFixedVolumeDevices = ...
                         AudioSystem.DEVICE_OUT_SPDIF;

In this case, the audio system always works as fixed volume device, and volume control can't work any more for HDMI output.

We think SPDIF is very trick as it doesn't support hotplug. To support it, we have to leave it as attached device.
But after that, our device will become a fixed volume device again.
So, I suggest to remove SPDIF from fixed volume device.

Bug: 136548998

Change-Id: I67841c6019085ce458ca496f3fd4b3b5dd12e5d3