Change the encryption feature to use a callback for decryption.

This allows code calling the library can choose an arbitrary
encryption algorithm.

Decoder control parameter VP8_SET_DECRYPT_KEY is renamed to
VP8D_SET_DECRYPTOR, and now takes an small config struct instead
of just a byte array.

Change-Id: I0462b3388d8d45057e4f79a6b6777fe713dc546e