Allow notification sound check from any thread.

// FREEBIE

Signal/src/environment/NotificationsManager.m

@@ -310,8 +310,8 @@
 
 - (BOOL)shouldPlaySoundForNotification
 {
-    OWSAssert([NSThread isMainThread]);
-
+    @synchronized(self)
+    {
         // Play no more than 2 notification sounds in a given
         // five-second window.
         const CGFloat kNotificationWindowSeconds = 5.f;
@@ -341,6 +341,7 @@
             DDLogDebug(@"Skipping sound for notification");
             return NO;
         }
+    }
 }
 
 #pragma mark - Util