Clean up output directory before copying over new files (#14085)

* Clean up output directory before copying over new files

* Use standard clean task command

packages/flutter_tools/gradle/flutter.gradle

@@ -321,6 +321,7 @@ class FlutterPlugin implements Plugin<Project> {
             Task copyFlxTask = project.tasks.create(name: "copyFlutterAssets${variant.name.capitalize()}", type: Copy) {
                 dependsOn flutterTask
                 dependsOn variant.mergeAssets
+                dependsOn "clean${variant.mergeAssets.name.capitalize()}"
                 into variant.mergeAssets.outputDir
                 with flutterTask.assets
             }
@@ -453,7 +454,9 @@ class FlutterTask extends BaseFlutterTask {
     CopySpec getAssets() {
         return project.copySpec {
             from "${intermediateDir}"
+
             include "flutter_assets/**" // the working dir and its files
+
             if (buildMode != 'debug') {
                 if (preferSharedLibrary) {
                     include "${intermediateDir}/app.so"