fix: Select first of identical audio streams (shaka-project#3869)

If a manifest lists 2 audio streams, select the first acceptable stream instead of the last one. For example below, previously the 2nd stream was selected, and now the first stream will be selected. Other players like roku, video.js and exoplayer select the first one.

```
#EXT-X-MEDIA:TYPE=AUDIO,URI="stream_1.m3u8",GROUP-ID="default-audio-group",NAME="128k",AUTOSELECT=YES,CHANNELS="2"
#EXT-X-MEDIA:TYPE=AUDIO,URI="stream_2.m3u8",GROUP-ID="default-audio-group",NAME="64k",CHANNELS="2"
(video streams snipped)
```

lib/abr/simple_abr_manager.js

@@ -9,7 +9,6 @@ goog.provide('shaka.abr.SimpleAbrManager');
 goog.require('goog.asserts');
 goog.require('shaka.abr.EwmaBandwidthEstimator');
 goog.require('shaka.log');
-goog.require('shaka.util.Iterables');
 goog.require('shaka.util.StreamUtils');
 
 
@@ -145,23 +144,30 @@ shaka.abr.SimpleAbrManager = class {
     // Start by assuming that we will use the first Stream.
     let chosen = sortedVariants[0] || null;
 
-    const enumerate = (it) => shaka.util.Iterables.enumerate(it);
-    for (const {item, next} of enumerate(sortedVariants)) {
+    for (let i = 0; i < sortedVariants.length; i++) {
+      const item = sortedVariants[i];
       const playbackRate =
           !isNaN(this.playbackRate_) ? Math.abs(this.playbackRate_) : 1;
       const itemBandwidth = playbackRate * item.bandwidth;
       const minBandwidth =
           itemBandwidth / this.config_.bandwidthDowngradeTarget;
-      const nextBandwidth =
-          playbackRate * (next || {bandwidth: Infinity}).bandwidth;
+      let next = {bandwidth: Infinity};
+      for (let j = i + 1; j < sortedVariants.length; j++) {
+        if (item.bandwidth != sortedVariants[j].bandwidth) {
+          next = sortedVariants[j];
+          break;
+        }
+      }
+      const nextBandwidth = playbackRate * next.bandwidth;
       const maxBandwidth = nextBandwidth / this.config_.bandwidthUpgradeTarget;
       shaka.log.v2('Bandwidth ranges:',
           (itemBandwidth / 1e6).toFixed(3),
           (minBandwidth / 1e6).toFixed(3),
           (maxBandwidth / 1e6).toFixed(3));
 
       if (currentBandwidth >= minBandwidth &&
-          currentBandwidth <= maxBandwidth) {
+          currentBandwidth <= maxBandwidth &&
+          chosen.bandwidth != item.bandwidth) {
         chosen = item;
       }
     }

test/abr/simple_abr_manager_unit.js

@@ -91,7 +91,7 @@ describe('SimpleAbrManager', () => {
     config.defaultBandwidthEstimate = 3e6;
     abrManager.configure(config);
     const chosen = abrManager.chooseVariant();
-    expect(chosen.id).toBe(104);
+    expect(chosen.id).toBe(103);
   });
 
   it('can handle empty variants', () => {
@@ -208,7 +208,7 @@ describe('SimpleAbrManager', () => {
     abrManager.segmentDownloaded(1000, bytesPerSecond);
 
     // Expect variants 4 to be chosen
-    const expectedVariant = variants[4];
+    const expectedVariant = variants[3];
 
     expect(switchCallback).toHaveBeenCalledWith(expectedVariant);
   });