Minor cleanup to BandwidthMeter/DefaultBandwidthMeter

- Improve variable naming
- In the edge case that bytes are transferred in a sample that
  has 0 elapsed time, carry the bytes forward into the next
  sample. If we don't do this then the estimate will be calculated
  as though those bytes were never transferred. In practice I
  suspect this very rarely occurs.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=218551208

library/core/src/main/java/com/google/android/exoplayer2/upstream/BandwidthMeter.java

@@ -29,30 +29,33 @@ public interface BandwidthMeter {
   interface EventListener {
 
     /**
-     * Called periodically to indicate that bytes have been transferred.
+     * Called periodically to indicate that bytes have been transferred or the estimated bitrate has
+     * changed.
      *
      * <p>Note: The estimated bitrate is typically derived from more information than just {@code
      * bytes} and {@code elapsedMs}.
      *
-     * @param elapsedMs The time taken to transfer the bytes, in milliseconds.
-     * @param bytes The number of bytes transferred.
-     * @param bitrate The estimated bitrate in bits/sec.
+     * @param elapsedMs The time taken to transfer {@code bytesTransferred}, in milliseconds. This
+     *     is at most the elapsed time since the last callback, but may be less if there were
+     *     periods during which data was not being transferred.
+     * @param bytesTransferred The number of bytes transferred since the last callback.
+     * @param bitrateEstimate The estimated bitrate in bits/sec.
      */
-    void onBandwidthSample(int elapsedMs, long bytes, long bitrate);
+    void onBandwidthSample(int elapsedMs, long bytesTransferred, long bitrateEstimate);
   }
 
-  /** Returns the estimated bandwidth in bits/sec. */
+  /** Returns the estimated bitrate. */
   long getBitrateEstimate();
 
   /**
    * Returns the {@link TransferListener} that this instance uses to gather bandwidth information
-   * from data transfers. May be null, if no transfer listener is used.
+   * from data transfers. May be null if the implementation does not listen to data transfers.
    */
   @Nullable
   TransferListener getTransferListener();
 
   /**
-   * Adds an {@link EventListener} to be informed of bandwidth samples.
+   * Adds an {@link EventListener}.
    *
    * @param eventHandler A handler for events.
    * @param eventListener A listener of events.

library/core/src/main/java/com/google/android/exoplayer2/upstream/DefaultBandwidthMeter.java

@@ -309,16 +309,16 @@ public final class DefaultBandwidthMeter implements BandwidthMeter, TransferList
           || totalBytesTransferred >= BYTES_TRANSFERRED_FOR_ESTIMATE) {
         bitrateEstimate = (long) slidingPercentile.getPercentile(0.5f);
       }
-    }
-    notifyBandwidthSample(sampleElapsedTimeMs, sampleBytesTransferred, bitrateEstimate);
-    if (--streamCount > 0) {
+      notifyBandwidthSample(sampleElapsedTimeMs, sampleBytesTransferred, bitrateEstimate);
       sampleStartTimeMs = nowMs;
-    }
-    sampleBytesTransferred = 0;
+      sampleBytesTransferred = 0;
+    } // Else any sample bytes transferred will be carried forward into the next sample.
+    streamCount--;
   }
 
-  private void notifyBandwidthSample(int elapsedMs, long bytes, long bitrate) {
-    eventDispatcher.dispatch(listener -> listener.onBandwidthSample(elapsedMs, bytes, bitrate));
+  private void notifyBandwidthSample(int elapsedMs, long bytesTransferred, long bitrateEstimate) {
+    eventDispatcher.dispatch(
+        listener -> listener.onBandwidthSample(elapsedMs, bytesTransferred, bitrateEstimate));
   }
 
   private long getInitialBitrateEstimateForNetworkType(@C.NetworkType int networkType) {