Fix NoSuchMethodError on CompletableFuture crash.

app/proguard/proguard.cfg

@@ -10,6 +10,7 @@
 -keepclassmembers class ** {
     public void onEvent*(**);
 }
+-keep class org.signal.libsignal.internal.CompletableFuture** { *; }
 
 # Protobuf lite
 -keep class * extends com.google.protobuf.GeneratedMessageLite { *; }