(Related to GitHub #1706)
When the timeline becomes empty, the playback state transitions to "ended".
When the timeline becomes non-empty again, exceptions are thrown because MSG_DO_SOME_WORK is still regularly sent and media periods are getting prepared.
This change ensures that no MSG_DO_SOME_WORK messages are sent in "ended" state.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160537147
