Fix crash when changing username on Android API < 24.

Basically, the crash was caused by using `Collections.unmodifiableList()`, which creates an `UnmodifiableCollection` wrapper around the original `List`.

That `UnmodifiableCollection` wrapper contains methods that return `java.util.stream.Stream` – which was added in Java 8, but can be used on Android API < 24 through [desugaring](https://developer.android.com/studio/write/java8-support-table). 

_However_, it appears that when Jackson is using reflection to serialize `ReserveUsernameRequest`, it examines all of the methods of that class. This results in a `java.lang.NoClassDefFoundError: java.util.stream.Stream`, because the reflection is looking at the actual runtime `UnmodifiableCollection` class and not desugared code.

As far as I can tell, that behavior is unavoidable and the only way to avoid this issue is to avoid using `Collections.unmodifiableList()` in classes that are serialized by Jackson on Android API < 24.

Author: jeffrey-signal

libsignal-service/src/main/java/org/whispersystems/signalservice/internal/push/ReserveUsernameRequest.java

@@ -2,15 +2,14 @@
 
 import com.fasterxml.jackson.annotation.JsonProperty;
 
-import java.util.Collections;
 import java.util.List;
 
 public class ReserveUsernameRequest {
   @JsonProperty
   private List<String> usernameHashes;
 
   public ReserveUsernameRequest(List<String> usernameHashes) {
-    this.usernameHashes = Collections.unmodifiableList(usernameHashes);
+    this.usernameHashes = usernameHashes;
   }
 
   List<String> getUsernameHashes() {