This removes the need to calculate the time needed to run the doSomeWork
method. Consequently, we can use both the real Clock/Handler and the
FakeClock without changing the way the playback loop works and without
violating the interfaces of Clock or Handler.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=180665647

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=180558741

This allows to inject a FakeClock for tests. Other playback components
(e.g. some media sources) still use SystemClock but they can be amended
in the future if needed.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179921889

Especially this removes the need for the Clock interface to directly
implement Handler methods. Instead, we have a separate Handler interface
and the FakeClock is able to construct such a Handler.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=179918255