Generate complete silent audio frames

`SilentAudioGenerator` could output a fractional audio frame, and this could cause downstream components to throw because of trying to read a complete audio frame but only seeing a partial one.

Calculate the output buffer size based on the frame size (which is a no-op for stereo 16-bit audio) and calculate a total number of frames to output then multiple by the frame size.

PiperOrigin-RevId: 494992941

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/SilentAudioGenerator.java

@@ -16,19 +16,21 @@
 
 package com.google.android.exoplayer2.transformer;
 
+import com.google.android.exoplayer2.C;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 
 /* package */ final class SilentAudioGenerator {
-  private static final int DEFAULT_BUFFER_SIZE = 4096;
+  private static final int DEFAULT_BUFFER_SIZE_FRAMES = 1024;
 
   private final ByteBuffer internalBuffer;
 
   private long remainingBytesToOutput;
 
   public SilentAudioGenerator(long totalDurationUs, long sampleRate, int frameSize) {
-    remainingBytesToOutput = (sampleRate * frameSize * totalDurationUs) / 1_000_000L;
-    internalBuffer = ByteBuffer.allocate(DEFAULT_BUFFER_SIZE).order(ByteOrder.nativeOrder());
+    remainingBytesToOutput = frameSize * ((sampleRate * totalDurationUs) / C.MICROS_PER_SECOND);
+    internalBuffer =
+        ByteBuffer.allocate(DEFAULT_BUFFER_SIZE_FRAMES * frameSize).order(ByteOrder.nativeOrder());
     internalBuffer.flip();
   }
 

testdata/src/test/assets/transformerdumps/mp4/sample.mp4.silentaudio.dump

@@ -370,8 +370,8 @@ sample:
   presentationTimeUs = 998459
 sample:
   trackIndex = 1
-  dataHashCode = -1029274849
-  size = 409
+  dataHashCode = -587391743
+  size = 408
   isKeyFrame = true
   presentationTimeUs = 1021679
 sample: