SDK / exoplayer

- Ensure that no memory is used by audio processors that are always inactive, by
  only allocating in flush() if active. If data was already allocated but a
  processor becomes inactive we assume that the allocation may be needed in
  future so do not remove it (e.g., in the case of ResamplingAudioProcessor).
- Make SilenceSkippingAudioProcessor set up its buffers in flush(), and clarify
  that it is always necessary to call flush() if configure() returns true.
- Make reset() reset all state for all processors.
- Use @Nullable state or empty arrays for inactive audio processor buffers.
- Miscellaneous style/consistency cleanup.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190895783