feat(permissions-android): export Rationale type and add missing properties according to docs (#23510)

Summary:
I checked the documentation over at https://facebook.github.io/react-native/docs/permissionsandroid and the documented properties `buttonPositive`, `buttonNegative` and `buttonNeutral` are not available in the flow-type definitions.

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Also the Rationale type is not exported which makes it hard to reuse it in a library or in your own application code.

However I do not know if it is actually intended to import "internal" flow-types from `react-native` since I could not find any other type or interface being exported. So I am not 100% sure if I should have done this.

[General] [Added] - Export Rationale flow-type and add missing properties `buttonPositive`, `buttonNegative` and `buttonNeutral` to the documentation.
Pull Request resolved: https://github.com/facebook/react-native/pull/23510

Differential Revision: D14123848

Pulled By: cpojer

fbshipit-source-id: 4040590932db645da6422d680246fed1d46dbe79

Libraries/PermissionsAndroid/PermissionsAndroid.js

@@ -12,9 +12,12 @@
 
 const NativeModules = require('NativeModules');
 
-type Rationale = {
+export type Rationale = {
   title: string,
   message: string,
+  buttonPositive?: string,
+  buttonNegative?: string,
+  buttonNeutral?: string,
 };
 
 type PermissionStatus = 'granted' | 'denied' | 'never_ask_again';