Support the action.devices.commands.mute intent to mute and unmute
media_players that declare support for mute/unmute.
For media players with support for volume up/down, but no support for
setting the volume to a specific number, allow use of the
action.devices.commands.relativeMute intent to control volume up/down.
This will improve support for IR blasters and other open-loop
media_player integrations.
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