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](https://github.com/Microsoft/react-native-windows/pull/1807)).

Thank you for sending the PR! We appreciate you spending the time to work on these changes.
Help us understand your motivation by explaining why you decided to make this change.

<!--
  Required: Write your motivation here.
  If this PR fixes an issue, type "Fixes #issueNumber" to automatically close the issue when the PR is merged.
-->

<!--
  Required: Write your test plan here. If you changed any code, please provide us with
  clear instructions on how you verified your changes work. Bonus points for screenshots and videos!
-->

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).

<!--
  Does this PR require a documentation change?
  Create a PR at https://github.com/facebook/react-native-website and add a link to it here.
-->

https://github.com/Microsoft/react-native-windows/pull/1807

<!--
  Required.
  Help reviewers and the release process by writing your own release notes. See below for an example.
-->

[CATEGORY] [TYPE] [LOCATION] - Message

<!--
  **INTERNAL and MINOR tagged notes will not be included in the next version's final release notes.**

    CATEGORY
  [----------]      TYPE
  [ CLI      ] [-------------]    LOCATION
  [ DOCS     ] [ BREAKING    ] [-------------]
  [ GENERAL  ] [ BUGFIX      ] [ {Component} ]
  [ INTERNAL ] [ ENHANCEMENT ] [ {Filename}  ]
  [ IOS      ] [ FEATURE     ] [ {Directory} ]   |-----------|
  [ ANDROID  ] [ MINOR       ] [ {Framework} ] - | {Message} |
  [----------] [-------------] [-------------]   |-----------|

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
-->

[GENERAL][Enhancement][View] - Make `ViewPropTypes` more friendly to platform extensions.
Closes https://github.com/facebook/react-native/pull/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,
 };