Touchable [nfc]: Inline value for background prop on Android.

After the upcoming RN v0.61 -> v0.62 upgrade, Flow would correctly
flag the fact that we're trying to compare a `number | string` value
to a number. (`Platform.Version` is a number on Android and a string
on iOS [1].)

With the new placement of the `Platform.Version` check, Flow will be
able to infer that `Platform.Version` is a number from the
conditional on `Platform.OS` above. (It looks like RN types
`Platform` as a union of types corresponding to each platform, so
`Platform.OS` and `Platform.Version` correctly correspond with each
other.)

[1] https://reactnative.dev/docs/platform-specific-code#detecting-the-android-version

src/common/Touchable.js

@@ -6,11 +6,6 @@ import type { ViewStyleProp } from 'react-native/Libraries/StyleSheet/StyleSheet
 import type { Node as React$Node } from 'react';
 import { HIGHLIGHT_COLOR } from '../styles';
 
-const androidBackground =
-  Platform.Version >= 21
-    ? TouchableNativeFeedback.Ripple(HIGHLIGHT_COLOR)
-    : TouchableNativeFeedback.SelectableBackground();
-
 type Props = $ReadOnly<{|
   accessibilityLabel?: string,
   style?: ViewStyleProp,
@@ -92,7 +87,11 @@ export default class Touchable extends PureComponent<Props> {
     return (
       <TouchableNativeFeedback
         accessibilityLabel={accessibilityLabel}
-        background={androidBackground}
+        background={
+          Platform.Version >= 21
+            ? TouchableNativeFeedback.Ripple(HIGHLIGHT_COLOR)
+            : TouchableNativeFeedback.SelectableBackground()
+        }
         onPress={onPress}
         onLongPress={onLongPress}
       >