BackHandler: specify function return type for handler (#28192)

Summary:
Following the comment DefinitelyTyped/DefinitelyTyped#42618 (comment)

Modify the flowtype of BackHandler function to have more specific return type.

## Changelog

[General] [Changed] - Changed type of BackHandler to be more specific.
Pull Request resolved: #28192

Test Plan: No flow error in RNTester

Reviewed By: TheSavior

Differential Revision: D20771113

Pulled By: hramos

fbshipit-source-id: 5ca65e2a2b3f8726b8fb4606473d8fad5b0ce730

Author: Naturalclar

Libraries/Utilities/BackHandler.android.js

@@ -63,11 +63,11 @@ type TBackHandler = {|
   +exitApp: () => void,
   +addEventListener: (
     eventName: BackPressEventName,
-    handler: Function,
+    handler: () => ?boolean,
   ) => {remove: () => void, ...},
   +removeEventListener: (
     eventName: BackPressEventName,
-    handler: Function,
+    handler: () => ?boolean,
   ) => void,
 |};
 const BackHandler: TBackHandler = {
@@ -87,7 +87,7 @@ const BackHandler: TBackHandler = {
    */
   addEventListener: function(
     eventName: BackPressEventName,
-    handler: Function,
+    handler: () => ?boolean,
   ): {remove: () => void, ...} {
     if (_backPressSubscriptions.indexOf(handler) === -1) {
       _backPressSubscriptions.push(handler);
@@ -102,7 +102,7 @@ const BackHandler: TBackHandler = {
    */
   removeEventListener: function(
     eventName: BackPressEventName,
-    handler: Function,
+    handler: () => ?boolean,
   ): void {
     if (_backPressSubscriptions.indexOf(handler) !== -1) {
       _backPressSubscriptions.splice(

Libraries/Utilities/BackHandler.ios.js

@@ -54,11 +54,11 @@ type TBackHandler = {|
   +exitApp: () => void,
   +addEventListener: (
     eventName: BackPressEventName,
-    handler: Function,
+    handler: () => ?boolean,
   ) => {remove: () => void, ...},
   +removeEventListener: (
     eventName: BackPressEventName,
-    handler: Function,
+    handler: () => ?boolean,
   ) => void,
 |};
 
@@ -93,7 +93,7 @@ if (Platform.isTV) {
 
     addEventListener: function(
       eventName: BackPressEventName,
-      handler: Function,
+      handler: () => ?boolean,
     ): {remove: () => void, ...} {
       _backPressSubscriptions.add(handler);
       return {
@@ -103,7 +103,7 @@ if (Platform.isTV) {
 
     removeEventListener: function(
       eventName: BackPressEventName,
-      handler: Function,
+      handler: () => ?boolean,
     ): void {
       _backPressSubscriptions.delete(handler);
     },

Libraries/Utilities/__mocks__/BackHandler.js

@@ -16,7 +16,7 @@ const BackHandler = {
 
   addEventListener: function(
     eventName: BackPressEventName,
-    handler: Function,
+    handler: () => ?boolean,
   ): {remove: () => void} {
     _backPressSubscriptions.add(handler);
     return {
@@ -26,7 +26,7 @@ const BackHandler = {
 
   removeEventListener: function(
     eventName: BackPressEventName,
-    handler: Function,
+    handler: () => ?boolean,
   ): void {
     _backPressSubscriptions.delete(handler);
   },