Fix various gradle paths for Windows (#8935)

packages/flutter_tools/lib/src/android/android_studio.dart

@@ -38,7 +38,9 @@ String get gradleExecutable {
   if (gradleDir != null) {
     if (fs.isFileSync(gradleDir))
       return gradleDir;
-    return fs.path.join(gradleDir, 'bin', 'gradle');
+    return fs.path.join(
+        gradleDir, 'bin', platform.isWindows ? 'gradle.bat' : 'gradle'
+    );
   }
   return androidStudio?.gradleExecutable ?? os.which('gradle')?.path;
 }

packages/flutter_tools/lib/src/android/gradle.dart

@@ -105,7 +105,9 @@ String locateSystemGradle({ bool ensureExecutable: true }) {
 }
 
 String locateProjectGradlew({ bool ensureExecutable: true }) {
-  final String path = 'android/gradlew';
+  final String path = fs.path.join(
+      'android', platform.isWindows ? 'gradlew.bat' : 'gradlew'
+  );
 
   if (fs.isFileSync(path)) {
     final File gradle = fs.file(path);