Post startService() in onResume() as a possible fix to an Android P bug.

Got confirmation that the crash we're seeing is a bug, and this might be
a possible workaround.

src/org/thoughtcrime/securesms/util/Util.java

@@ -384,6 +384,10 @@ public class Util {
     }
   }
 
+  public static void postToMain(final @NonNull Runnable runnable) {
+    handler.post(runnable);
+  }
+
   public static void runOnMain(final @NonNull Runnable runnable) {
     if (isMainThread()) runnable.run();
     else                handler.post(runnable);