SDK / exoplayer

This includes a metric logger class which logs events and generates metrics, and some changes to the Perfgate benchmark config to add the metrics. So far, only rebuffer counts, rebuffer time and startup time are measured, together with auxiliary values of total playback time used for testing and number of playback failures.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161513672