Update clean command in devicelab tests (#12640)

dev/devicelab/bin/tasks/flutter_gallery_instrumentation_test.dart

@@ -19,7 +19,7 @@ Future<Null> main() async {
       final Device device = await devices.workingDevice;
       await device.unlock();
       await flutter('packages', options: <String>['get']);
-      await flutter('build', options: <String>['clean']);
+      await flutter('clean');
       await flutter('build', options: <String>['apk', '--target', 'test/live_smoketest.dart']);
       final String androidStudioPath = grep('Android Studio at', from: await evalFlutter('doctor')).first.split(' ').last;
       await exec('./tool/run_instrumentation_test.sh', <String>[], environment: <String, String>{

dev/devicelab/lib/tasks/perf_tests.dart

@@ -192,7 +192,7 @@ class BuildTest {
   }
 
   static Future<Map<String, dynamic>> _buildAot(bool previewDart2) async {
-    await flutter('build', options: <String>['clean']);
+    await flutter('clean');
     final Stopwatch watch = new Stopwatch()..start();
     final List<String> options = <String>[
       'aot',
@@ -218,7 +218,7 @@ class BuildTest {
   }
 
   static Future<Map<String, dynamic>> _buildDebug(bool previewDart2) async {
-    await flutter('build', options: <String>['clean']);
+    await flutter('clean');
 
     final Stopwatch watch = new Stopwatch();
     if (deviceOperatingSystem == DeviceOperatingSystem.ios) {

dev/devicelab/lib/tasks/size_tests.dart

@@ -29,7 +29,7 @@ TaskFunction createBasicMaterialAppSizeTest() {
 
       await inDirectory(sampleDir, () async {
         await flutter('packages', options: <String>['get']);
-        await flutter('build', options: <String>['clean']);
+        await flutter('clean');
 
         if (deviceOperatingSystem == DeviceOperatingSystem.ios) {
           await prepareProvisioningCertificates(sampleDir.path);