ao_opensles: bump device buffer size to 250ms

Although half (non-fast track on sink rate) or one-third (non-fast track not on sink rate) of the buffer size of the created AudioTrack instance as the SL Enqueue buffer size is basically enough for dropout-free playback, only using the full size can avoid stutter upon (re)start of playback.

Here are the various buffer sizes on different track/sink rate when on Bluetooth audio on Android O:

aptX @ 48kHz:
Sink rate: 48000 Hz
44100 Hz: 10632 frames (241.09 ms)
48000 Hz: 11544 frames (240.50 ms)
88200 Hz: 21216 frames (240.54 ms)
96000 Hz: 23088 frames (240.50 ms)
176400 Hz: 42384 frames (240.27 ms)
192000 Hz: 46128 frames (240.25 ms)

SBC/AAC/aptX @ 44.1kHz:
Sink rate: 44100 Hz
44100 Hz: 10776 frames (244.35 ms)
48000 Hz: 11748 frames (244.75 ms)
88200 Hz: 21552 frames (244.35 ms)
96000 Hz: 23448 frames (244.25 ms)
176400 Hz: 43056 frames (244.08 ms)
192000 Hz: 46848 frames (244.00 ms)

The above results were produced with the following code:

import android.media.AudioAttributes;
import android.media.AudioFormat;
import android.media.AudioTrack;

class AudioInfo {
    public static void main(String[] args) {
	int nosr = AudioTrack.getNativeOutputSampleRate(3);
	System.out.printf("Sink rate: %d Hz\n", nosr);

	int[] rates = {44100,48000,88200,96000,176400,192000};
	for (int rate: rates) {
	    AudioAttributes aa = new AudioAttributes.Builder().setFlags(256).build();
	    AudioFormat af = new AudioFormat.Builder().setSampleRate(rate).build();
	    AudioTrack at = new AudioTrack(aa, af, 4, 1, 0);
	    int sr = at.getSampleRate();
	    int bs = at.getBufferSizeInFrames();
	    float ms = bs * (float) 1000 / sr;
	    at.release();
	    System.out.printf("%d Hz: %d frames (%.2f ms)\n", sr, bs, ms);
	}
    }
}

Therefore bumping the device buffer size to 250ms.

audio/out/ao_opensles.c

@@ -100,7 +100,7 @@ static void buffer_callback(SLBufferQueueItf buffer_queue, void *context)
     pthread_mutex_unlock(&p->buffer_lock);
 }
 
-#define DEFAULT_BUFFER_SIZE_MS 200
+#define DEFAULT_BUFFER_SIZE_MS 250
 
 #define CHK(stmt) \
     { \