Support platform-specific prop type overrides

Summary:
Moves `PlatformViewPropTypes` to the end of `ViewPropTypes` so platforms can override prop type values. An example of this is in `react-native-windows`, we want to support additional values for the `importantForAccessibility` prop type that are specific to Windows (more details in [this PR](microsoft/react-native-windows#1807)).

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Run jest tests, test RNTester on iOS and Android. Did not test tvOS, but this should not impact tvOS as the only props included from `PlatformViewPropTypes` are unique (i.e., are not overridden in this file).

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microsoft/react-native-windows#1807

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[GENERAL][Enhancement][View] - Make `ViewPropTypes` more friendly to platform extensions.
Closes facebook#19090

Differential Revision: D7846438

Pulled By: shergin

fbshipit-source-id: e51ee3256d1c21001b371bd07da21319f3ecd810

Libraries/Components/View/ViewPropTypes.js

@@ -73,8 +73,6 @@ export type ViewProps = {
 } & TVViewProps;
 
 module.exports = {
-  ...PlatformViewPropTypes,
-
   /**
    * When `true`, indicates that the view is an accessibility element.
    * By default, all the touchable elements are accessible.
@@ -427,4 +425,9 @@ module.exports = {
    * See http://facebook.github.io/react-native/docs/view.html#needsoffscreenalphacompositing
    */
   needsOffscreenAlphaCompositing: PropTypes.bool,
+
+  /**
+   * Any additional platform-specific view prop types, or prop type overrides.
+   */
+  ...PlatformViewPropTypes,
 };