Update path to built .apk. (#8137)

dev/devicelab/lib/tasks/size_tests.dart

diff --git a/dev/devicelab/lib/tasks/size_tests.dart b/dev/devicelab/lib/tasks/size_tests.dart
index f906cae..5f0b089 100644
--- a/dev/devicelab/lib/tasks/size_tests.dart
+++ b/dev/devicelab/lib/tasks/size_tests.dart
@@ -36,7 +36,7 @@
           releaseSizeInBytes = await file('${sampleDir.path}/build/app.ipa').length();
         } else {
           await flutter('build', options: <String>['apk', '--release']);
-          releaseSizeInBytes = await file('${sampleDir.path}/build/app.apk').length();
+          releaseSizeInBytes = await file('${sampleDir.path}/android/app/build/outputs/apk/app.apk').length();
         }
       });
     });