QoE Metrics Reporting - Notify Media Session Handler about start of media playback

[dsilhavy]

Feature description
As defined in 26.501 Section 5.5.3 step 11: The Media Player notifies the Media Session Handler of the start of media playback, causing the Media Session Handler to initialize and begin countdown of separate metrics reporting interval timers for the supported schemes.

For that reason, we need:

A message that is sent by the Media Stream Handler via IPC to the Media Session Handler to notify about the start of media playback

[dsilhavy]

Implemented in #67 . We use the Player.STATE_READY event from the Exoplayer.