Remove type union in PickeriOS/PickerNativeComponent

Summary:
This builds on the last diff to remove type a type union from Picker. This diff focuses on Picker internals.

Changelog: [JS] Remove type union in PickeriOS/PickerNativeComponent

Reviewed By: sammy-SC

Differential Revision: D24254615

fbshipit-source-id: f788a2e123135c1e8b9909870c40f53b2dea0227

Author: Peter Argany

Libraries/Components/Picker/PickerIOS.ios.js

@@ -37,7 +37,7 @@ type PickerIOSChangeEvent = SyntheticEvent<
 
 type RCTPickerIOSItemType = $ReadOnly<{|
   label: ?Label,
-  value: ?(number | string),
+  value: ?string,
   textColor: ?ProcessedColorValue,
 |}>;
 
@@ -49,7 +49,7 @@ type Props = $ReadOnly<{|
   itemStyle?: ?TextStyleProp,
   onChange?: ?(event: PickerIOSChangeEvent) => mixed,
   onValueChange?: ?(itemValue: string | number, itemIndex: number) => mixed,
-  selectedValue: ?(number | string),
+  selectedValue: ?string,
   accessibilityLabel?: ?string,
 |}>;
 
@@ -60,7 +60,7 @@ type State = {|
 
 type ItemProps = $ReadOnly<{|
   label: ?Label,
-  value?: ?(number | string),
+  value?: ?string,
   color?: ?ColorValue,
 |}>;
 

Libraries/Components/Picker/RCTPickerNativeComponent.js

@@ -28,7 +28,7 @@ type PickerIOSChangeEvent = SyntheticEvent<
 
 type RCTPickerIOSItemType = $ReadOnly<{|
   label: ?Label,
-  value: ?(number | string),
+  value: ?string,
   textColor: ?ProcessedColorValue,
 |}>;