fix missing selection indicator lines (#18885)

Summary:
This PR is based on #13342 by pvinis and fixes #14442.

As suggested in the discussion on the PR mentioned above, I moved the code from `React/Views/RCTPickerManager.m` to the `initWithFrame` function in `React/Views/RCTPicker.m` and verified that it still fixes the problem.

Before the change in this PR the selection indicator lines are missing when the Picker is initially added to the View and only appear after changing to a different Tab and back. This happens both in the Simulator and my real device (iPhone 6S on iOS 11.3).

![beforechange](https://user-images.githubusercontent.com/7568362/38824104-7b294cae-41a8-11e8-8609-7a647ab3adb8.png)

After the change, the indicator lines always appear. I did not notice any side effects of this change when playing around with the Picker in different configurations.

![afterchange](https://user-images.githubusercontent.com/7568362/38824109-82a5b382-41a8-11e8-8af3-ca07c8b2c30e.png)

#13342 also fixes this issue but appears to be inactive.

[IOS] [BUGFIX] [PickerIOS] - Fixed missing selection indicator lines
Pull Request resolved: #18885

Differential Revision: D8945483

Pulled By: hramos

fbshipit-source-id: 2b6c6f42559691530b062503feb24bc305f4a8af

Author: VSchlattinger

React/Views/RCTPicker.m

@@ -23,6 +23,7 @@ - (instancetype)initWithFrame:(CGRect)frame
     _selectedIndex = NSNotFound;
     _textAlign = NSTextAlignmentCenter;
     self.delegate = self;
+    [self selectRow:0 inComponent:0 animated:YES]; // Workaround for missing selection indicator lines (see https://stackoverflow.com/questions/39564660/uipickerview-selection-indicator-not-visible-in-ios10)
   }
   return self;
 }