[path_provider] Remove use of Pigeon's Dart test generator

- Updates `path_provider_android` to use DI to inject the fake Pigeon
  API implementation, instead of the Dart test generator, following the
  pattern of newer plugins in the repo.
- Does the same for `path_provider_foundation`, and also replaces the
  use of mockito with a trivial fake, to simplify test maintenance.

Part of flutter/flutter#159886

Author: stuartmorgan-g

packages/path_provider/path_provider_android/lib/path_provider_android.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart' show visibleForTesting;
 import 'package:path_provider_platform_interface/path_provider_platform_interface.dart';
 import 'messages.g.dart' as messages;
 
@@ -36,7 +37,11 @@ messages.StorageDirectory _convertStorageDirectory(
 
 /// The Android implementation of [PathProviderPlatform].
 class PathProviderAndroid extends PathProviderPlatform {
-  final messages.PathProviderApi _api = messages.PathProviderApi();
+  /// Creates an instance of [PathProviderAndroid].
+  PathProviderAndroid({@visibleForTesting messages.PathProviderApi? api})
+    : _api = api ?? messages.PathProviderApi();
+
+  final messages.PathProviderApi _api;
 
   /// Registers this class as the default instance of [PathProviderPlatform].
   static void registerWith() {

packages/path_provider/path_provider_android/pigeons/messages.dart

@@ -13,7 +13,6 @@ import 'package:pigeon/pigeon.dart';
       package: 'io.flutter.plugins.pathprovider',
     ),
     dartOut: 'lib/messages.g.dart',
-    dartTestOut: 'test/messages_test.g.dart',
     copyrightHeader: 'pigeons/copyright.txt',
   ),
 )
@@ -31,7 +30,7 @@ enum StorageDirectory {
   documents,
 }
 
-@HostApi(dartHostTestHandler: 'TestPathProviderApi')
+@HostApi()
 abstract class PathProviderApi {
   @TaskQueue(type: TaskQueueType.serialBackgroundThread)
   String? getTemporaryPath();