SDK / exoplayer

- The fmp4 extractor was reading from sampleEncryptionData even
  for not-encrypted samples, which I'm pretty sure isn't right.
  Fixed this. For all encrypted content I've seen, this change
  will be a no-op because isEncrypted is always true if there's
  an encryptionBox present.
- Made webm extractor only set cryptoInfo if isEncrypted is true.
- Align variable naming in the two extractors, for clarity.