MediaCodecNdk: use AMediaCodec_setAsyncNotifyCallback on Android P
VLC's MediaCodec module is currently asynchronous since it is using a specific thread to pull output buffers. However, this is not a durable solution because nothing in the API/Documentation mention that it's safe to call 2 different MediaCodec methods from 2 different threads. For now, it has always worked (tested and checked the implementation from Android 4.1 to Android 8.0). But it's safer to use the new call AMediaCodec_setAsyncNotifyCallback instead in order to be future proof.
TODO: Use new asynchronous callbacks only if the "AMediaCodec_setAsyncNotifyCallback" symbol is found, otherwise fallback to old OutputThread.