Add metadata keys to communicate encrypted fragments and associated info.

    kKeyECM               = 'ecm ',  // raw data

    kKeyIsADTS            = 'adts',  // bool (int32_t)

    // If a MediaBuffer's data represents (at least partially) encrypted

    // data, the following fields aid in decryption.

    // The data can be thought of as pairs of plain and encrypted data

    // fragments, i.e. plain and encrypted data alternate.

    // The first fragment is by convention plain data (if that's not the

    // case, simply specify plain fragment size of 0).

    // kKeyEncryptedSizes and kKeyPlainSizes each map to an array of

    // size_t values. The sum total of all size_t values of both arrays

    // must equal the amount of data (i.e. MediaBuffer's range_length()).

    // If both arrays are present, they must be of the same size.

    // If only encrypted sizes are present it is assumed that all

    // plain sizes are 0, i.e. all fragments are encrypted.

    // To programmatically set these array, use the MetaData::setData API, i.e.

    // const size_t encSizes[];

    // meta->setData(

    //  kKeyEncryptedSizes, 0 /* type */, encSizes, sizeof(encSizes));

    // A plain sizes array by itself makes no sense.

    kKeyEncryptedSizes    = 'encr',  // size_t[]

    kKeyPlainSizes        = 'plai',  // size_t[]

    kKeyCryptoKey         = 'cryK',  // uint8_t[16]

    kKeyCryptoIV          = 'cryI',  // uint8_t[16]

};

enum {