Related PR : #5691
The result of `TopKSelector` may be wrong when `trim()` is invoked and quiksort fallback to `Arrasys.sort()`. Because `Arrasys.sort()` is been used by mistake there.
Following test case can trigger this bug.
```
{
    int n = 10000;
    int k = 10000;
    int testIteration = 10;
    Random random = new Random(System.currentTimeMillis());
    for (int iter = 0; iter < testIteration; iter ++) {
      // target array to be sorted using TopKSelector
      List<Integer> target = new ArrayList<>();
      for (int i = 0; i < 9; i++) {
        List<Integer> sortedArray = new ArrayList();
        for (int j = 0; j < n; j++) {
          sortedArray.add(random.nextInt());
        }
        sortedArray.sort(Integer::compareTo);
        target.addAll(sortedArray);
      }

      TopKSelector<Integer> top = TopKSelector.least(k, Integer::compareTo);
      for (int value : target) {
        top.offer(value);
      }

      target.sort(Integer::compareTo);
      assertEquals(top.topK(), target.subList(0, k));
    }
  }
```
}
  public void testZeroK() {
    TopKSelector<Integer> top = TopKSelector.least(0);
    for (int i = 0; i < 10; i++) {
      top.offer(i);
    }
    assertThat(top.topK()).isEmpty();
  }
  public void testNoElementsOffered() {
    TopKSelector<Integer> top = TopKSelector.least(10);
    assertThat(top.topK()).isEmpty();
  }
  public void testOfferedFewerThanK() {
    TopKSelector<Integer> top = TopKSelector.least(10);
    top.offer(3);
    top.offer(5);
    top.offer(2);
    assertThat(top.topK()).containsExactly(2, 3, 5).inOrder();
  }
  public void testOfferedKPlusOne() {
    for (List<Integer> list : Collections2.permutations(Ints.asList(1, 2, 3, 4, 5))) {
      TopKSelector<Integer> top = TopKSelector.least(4);
      top.offerAll(list);
      assertThat(top.topK()).containsExactly(1, 2, 3, 4).inOrder();
    }
  }
  public void testOfferedThreeK() {
    for (List<Integer> list : Collections2.permutations(Ints.asList(1, 2, 3, 4, 5, 6))) {
      TopKSelector<Integer> top = TopKSelector.least(2);
      top.offerAll(list);