Add current track info handling to Simple Audioservice

[krisgesling]

Description

Small addition to the Simple Audioservice to add current track info handling in line with current Audioservice definition.

How to test

With the simple audioservice as your default backend. Modify a Skill that plays audio, like the News Skill. While playing request track info:

self.audioservice.track_info()

Or using the Message directly: mycroft.audio.service.track_info

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• [x] CLA

[devops-mycroft]

Voight Kampff Integration Test Succeeded (Results)