The adaptive period currently extends the base (non-adaptive) period
to share common MediaPeriod boilerplate code.
However, once we start using the real SampleQueue in FakeMediaPeriod
the common code becomes even less and the overhead to support
multiple stream implementation from the base class is no longer
worth it. Thus, this change removes the class hierarchy and copies
the common parts to FakeAdaptiveMediaPeriod.
PiperOrigin-RevId: 347990468