iOS: Remove leading space from accessibilityLabel

In some cases, the accessibilityLabel contains a leading space. This is because `RCTRecursiveAccessibilityLabel` adds a space before every iteration of the loop including the first.

After this change, the contract is that:
  - `RCTRecursiveAccessibilityLabel` always returns a string with a leading space.
  - `accessibilityLabel` never returns a string with a leading space.

**Test plan**

I created a test app with the following code:

```
<View style={{height: 100, width: 100, backgroundColor: 'steelblue'}} accessible={true}>
  <View style={{height: 20, width: 20, backgroundColor: 'red'}} accessibilityLabel='One' />
  <View style={{height: 20, width: 20, backgroundColor: 'yellow'}} accessibilityLabel='Two' />
  <View style={{height: 20, width: 20, backgroundColor: 'green'}} accessibilityLabel='Three' />
</View>
```

Before this change, the accessibilityLabel of the outermost View was " One Two Three" (notice the leading space).

After this change, it is "One Two Three" as desired.

React/Views/RCTView.m

@@ -141,7 +141,8 @@ - (NSString *)accessibilityLabel
   if (super.accessibilityLabel) {
     return super.accessibilityLabel;
   }
-  return RCTRecursiveAccessibilityLabel(self);
+  NSString *str = RCTRecursiveAccessibilityLabel(self);
+  return [str stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
 }
 
 - (void)setPointerEvents:(RCTPointerEvents)pointerEvents