Read port configuration from UserDefaults

Summary:
This allows us to configure a simulator to use a non-default port pair by using the `defaults` mechanism. For instance, this can be done by running

```
xcrun simctl spawn booted defaults write "Apple Global Domain" "com.facebook.flipper.ports" -string "9088,9089"
```

Reviewed By: lblasa, timur-valiev

Differential Revision: D30731874

fbshipit-source-id: 689d60b1c392f36dceef1b3c3cfa0c88f54a7a82

iOS/FlipperKit/SKEnvironmentVariables.m

@@ -35,7 +35,14 @@ static int const DEFAULT_SECURE_PORT = 8088;
   return envInt > 0 ? envInt : fallback;
 }
 + (NSString*)getFlipperPortsVariable {
+  // Try to retrieve from environment first.
   NSString* value = NSProcessInfo.processInfo.environment[@"FLIPPER_PORTS"];
+  // If empty, check defaults instead.
+  if ([value length] == 0) {
+    value = [[NSUserDefaults standardUserDefaults]
+        stringForKey:@"com.facebook.flipper.ports"];
+  }
+
   return value;
 }
 @end