* Add play/pause functionality to vizio integration
Leverages existing pyvizio functionality. My impression is that it also
works for soundbars based on https://github.com/exiva/Vizio_SmartCast_API/issues/19.
* Set vizio assumed_state to True
The Vizio API is only capable of indicating whether the device is on
or off and not whether it's playing/paused/idle. Setting assumed_state
to True gives us separate Play and Pause buttons versus the (useless)
merged Play/Pause button we would get otherwise.
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