Add background color to iOS diagnostic screen

Summary:
Some apps have a default background color, for example black, making the diagnostic screen fully black.
Fix this by making sure the bg is always white so the text is readable.

Reviewed By: passy

Differential Revision: D9400682

fbshipit-source-id: a45744f833cb7fe5ca7db4dcdacc09f304670032

iOS/SonarKit/FlipperDiagnosticsViewController.m

@@ -20,6 +20,7 @@
   [self.scrollView addSubview:self.stateLabel];
   self.scrollView.contentSize = self.stateLabel.frame.size;
   [self.view addSubview:self.scrollView];
+  self.view.backgroundColor = [UIColor whiteColor];
 }
 
 - (void)onUpdate {