public static KeyTemplate createAesGcmKeyTemplate(int keySize) {
    AesGcmKeyFormat format = AesGcmKeyFormat.newBuilder()
        .setKeySize(keySize)
        .build();
    return KeyTemplate.newBuilder()
        .setValue(format.toByteString())
        .setTypeUrl(AesGcmKeyManager.getKeyType())
        .setOutputPrefixType(OutputPrefixType.TINK)
        .build();
  }
@return a {@link KeyTemplate} containing a {@link AesGcmKeyFormat} with some specified
    parameters.
java_src/src/main/java/com/google/crypto/tink/aead/AeadKeyTemplates.java