Add constructor to DefaultShuffleOrder to support sideloaded shuffle orders

Issue: #4915

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=221075615

library/core/src/main/java/com/google/android/exoplayer2/source/ShuffleOrder.java

@@ -55,6 +55,17 @@ public interface ShuffleOrder {
       this(length, new Random(randomSeed));
     }
 
+    /**
+     * Creates an instance with a specified shuffle order and the specified random seed. The random
+     * seed is used for {@link #cloneAndInsert(int, int)} invocations.
+     *
+     * @param shuffledIndices The shuffled indices to use as order.
+     * @param randomSeed A random seed.
+     */
+    public DefaultShuffleOrder(int[] shuffledIndices, long randomSeed) {
+      this(Arrays.copyOf(shuffledIndices, shuffledIndices.length), new Random(randomSeed));
+    }
+
     private DefaultShuffleOrder(int length, Random random) {
       this(createShuffledList(length, random), random);
     }

library/core/src/test/java/com/google/android/exoplayer2/source/ShuffleOrderTest.java

@@ -79,6 +79,23 @@ public final class ShuffleOrderTest {
     }
   }
 
+  @Test
+  public void testSideloadedShuffleOrder() {
+    int[] shuffledIndices = new int[] {2, 1, 0, 4, 3};
+    ShuffleOrder shuffleOrder = new DefaultShuffleOrder(shuffledIndices, RANDOM_SEED);
+    assertThat(shuffleOrder.getFirstIndex()).isEqualTo(2);
+    assertThat(shuffleOrder.getLastIndex()).isEqualTo(3);
+    for (int i = 0; i < 4; i++) {
+      assertThat(shuffleOrder.getNextIndex(shuffledIndices[i])).isEqualTo(shuffledIndices[i + 1]);
+    }
+    assertThat(shuffleOrder.getNextIndex(3)).isEqualTo(C.INDEX_UNSET);
+    for (int i = 4; i > 0; i--) {
+      assertThat(shuffleOrder.getPreviousIndex(shuffledIndices[i]))
+          .isEqualTo(shuffledIndices[i - 1]);
+    }
+    assertThat(shuffleOrder.getPreviousIndex(2)).isEqualTo(C.INDEX_UNSET);
+  }
+
   private static void assertShuffleOrderCorrectness(ShuffleOrder shuffleOrder, int length) {
     assertThat(shuffleOrder.getLength()).isEqualTo(length);
     if (length == 0) {