Fixed Flaky test found in UserDataTest

[sliu-UIUC]

Specifically, the tests comes from this line

    assertEquals(userData.getExternalIds(), externalIds.subList(0, 2));

in test test/java/com/facebook/ads/sdk/serverside/UserDataTest.java, which is a flaky test that
either pass or fail occasionally on its own. The reason is that externalIds is declared earlier as

 List<String> externalIds = Arrays.asList("external-1", "external-2", "external-2");

which ensures externalIds.subList(0, 2)) to always yield {"external-1", "external-2"} and in this
order. However, both userData.externalIds or userData.setExternalIds which set
the external ids field of user data utilize the dedup function defined in src/main/java/com/facebook/ads/sdk/serverside/UserData.java. Because dedup utilizes a hash set
data structure, external id values are not necessarily having a fixed order each time the
function is called, so both userData.externalIds or userData.setExternalIds can either
return {"external-1", "external-2"} or "{external-2", "external-1}" during different runs.

  private List<String> dedup(List<String> values) {
    if (isListNullOrEmpty(values)) return null;
    Set<String> set = new HashSet();
    for (String str : values) {
      if (str != null) {
        set.add(str);
      }
    }
    return new ArrayList(set);
  }

In the unit test, I stored the result of userData.getExternalIds in a list and sort the result
before hand, and the assertion should also yield true with the fix.

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