Feature/metrics reporting

[dsilhavy]

This PR adds support for QoE Metrics Reporting. In addition, it refactors the code to make it more modular, use dependency injection and introduce a more clear separation of concerns.

• Adds QoeMetricsReporter interface defining functions that need to be implemented for eachQoeMetricsReporter class. The idea is to have different handlers for each scheme. Moreover, applications can implement this interface to define their own QoE Metrics Reporting class.
• Adds QoeMetricsReporterExoplayer that implements QoeMetricsReporter for the default 3GPP scheme urn:3GPP:ns:PSS:DASH:QM10 using Exoplayer.
• Adds QoEMetricsReportingController to manage the available instances of QoeMetricsReporter. Currently, one reporting class per scheme is supported and can be registered via setQoeMetricsReporterForScheme. Once a QoEMetricsReport is requested by the MediaSessionHandler the QoEMetricsReportingController identifies the right QoeMetricsReporter for the desired scheme and calls getQoeMetricsReport on this instance.
• The ExoPlayerListener class dispatches Exoplayer events via an internal event bus. By registering for specific events the QoeMetricsReporter classes gain access to player related information such as quality switches.
• Dedicated service classes for incoming and outgoing messages
• Dedicated controller classes to handle use-case specific logic
• Increase version number to 1.2.0