Remove type union in Picker.js

Summary:
Flow type unions don't play well with Fabric components. This diff removes a union in `Picker.js` and fixes all the flow errors.

Before this diff, all these surfaces would crash with the new Fabric Picker impl, because the impl asserts that this field is a string.

Reviewed By: sammy-SC

Differential Revision: D24236317

fbshipit-source-id: 6e646c84fcd16658aaabe5e93507f5f33b346a65

Author: Peter Argany

Libraries/Components/Picker/Picker.js

@@ -30,9 +30,9 @@ type PickerItemProps = $ReadOnly<{|
 
   /**
    * The value to be passed to picker's `onValueChange` callback when
-   * this item is selected. Can be a string or an integer.
+   * this item is selected.
    */
-  value?: ?(number | string),
+  value?: ?string,
 
   /**
    * Color of this item's text.
@@ -62,9 +62,9 @@ type PickerProps = $ReadOnly<{|
   style?: ?TextStyleProp,
 
   /**
-   * Value matching value of one of the items. Can be a string or an integer.
+   * Value matching value of one of the items.
    */
-  selectedValue?: ?(number | string),
+  selectedValue?: ?string,
 
   /**
    * Callback for when an item is selected. This is called with the following parameters:

packages/rn-tester/js/examples/Modal/ModalExample.js

@@ -79,7 +79,7 @@ class ModalExample extends React.Component<{...}, $FlowFixMeState> {
     modalVisible: false,
     transparent: false,
     presentationStyle: 'fullScreen',
-    selectedSupportedOrientation: 0,
+    selectedSupportedOrientation: '0',
     currentOrientation: 'unknown',
   };
 
@@ -130,7 +130,7 @@ class ModalExample extends React.Component<{...}, $FlowFixMeState> {
           onRequestClose={() => this._setModalVisible(false)}
           supportedOrientations={
             supportedOrientationsPickerValues[
-              this.state.selectedSupportedOrientation
+              Number(this.state.selectedSupportedOrientation)
             ]
           }
           onOrientationChange={evt =>
@@ -222,12 +222,12 @@ class ModalExample extends React.Component<{...}, $FlowFixMeState> {
               this.setState({selectedSupportedOrientation: i})
             }
             itemStyle={styles.pickerItem}>
-            <Item label="Portrait" value={0} />
-            <Item label="Landscape" value={1} />
-            <Item label="Landscape left" value={2} />
-            <Item label="Portrait and landscape right" value={3} />
-            <Item label="Portrait and landscape" value={4} />
-            <Item label="Default supportedOrientations" value={5} />
+            <Item label="Portrait" value={'0'} />
+            <Item label="Landscape" value={'1'} />
+            <Item label="Landscape left" value={'2'} />
+            <Item label="Portrait and landscape right" value={'3'} />
+            <Item label="Portrait and landscape" value={'4'} />
+            <Item label="Default supportedOrientations" value={'5'} />
           </Picker>
         </View>
       </View>