[web] Change display mode of PWA default to standalone (#56056)

Change display mode of PWA default to standalone, update integration test manifest to new default

dev/integration_tests/flutter_gallery/web/manifest.json

@@ -2,7 +2,7 @@
     "name": "flutter_gallery",
     "short_name": "flutter_gallery",
     "start_url": ".",
-    "display": "minimal-ui",
+    "display": "standalone",
     "background_color": "#0175C2",
     "theme_color": "#0175C2",
     "description": "A new Flutter project.",

packages/flutter_tools/templates/app/web/manifest.json.tmpl

@@ -2,7 +2,7 @@
     "name": "{{projectName}}",
     "short_name": "{{projectName}}",
     "start_url": ".",
-    "display": "minimal-ui",
+    "display": "standalone",
     "background_color": "#0175C2",
     "theme_color": "#0175C2",
     "description": "{{description}}",