Update metrics tracker to most recent specifications

[defagos]

Description

This PR updates our metrics tracker implementation so that it conforms to the latest version of our specifications. After having identified a few issues during the implementation I also realized that having no UTs for our metrics tracker implementation was risky. I therefore introduced the necessary testing infrastructure so that we can ensure our implementation stays reliable over time.

Changes made

• Avoid stop event being sent when no start event has been sent.
• Use metric event date as date sent to the monitoring service.
• Use lower-case UUIDs which are more standard, improving consistency with UUIDs delivered by Android and the web.
• Send bitrate and bandwidth in bits per second.
• Add schema version.
• Group stall information.
• Rename key for storage of QoE metrics.
• Implement heartbeats.
• Revisit metrics tracker configuration to support optional heartbeat interval customization.
• Introduce testing infrastructure and improve consistency with the similar formalism existing for analytics tracker testing.
• Fix PillarboxMonitoring scheme.
• Fix MB calculation.
• Cache device type in comScore analytics.
• Group files in a better way in Monitoring and Analytics packages.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).

[defagos]

Thank you very much for the review!