Rename 'application' back to 'module', and make 'app' the default again for templates. (#22888)

We decided that redefining the default for templates was premature. We're going to go back to having "module" in experimental land again, and we'll try again when we have the feature set fully baked.

This keeps the writing of the .metadata files, and writing the template type to them, because that was a good improvement, and there are still a bunch of added tests that improve our coverage.

packages/flutter_tools/lib/src/flutter_manifest.dart

diff --git a/packages/flutter_tools/lib/src/flutter_manifest.dart b/packages/flutter_tools/lib/src/flutter_manifest.dart
index 7e4068e..bb731d8 100644
--- a/packages/flutter_tools/lib/src/flutter_manifest.dart
+++ b/packages/flutter_tools/lib/src/flutter_manifest.dart
@@ -110,14 +110,14 @@
     return _flutterDescriptor['uses-material-design'] ?? false;
   }
 
-  /// True if this manifest declares a Flutter application project.
+  /// True if this manifest declares a Flutter module project.
   ///
-  /// A Flutter project is considered an application when it has a `application:`
-  /// descriptor. A Flutter application project supports integration into an
-  /// existing host app.
+  /// A Flutter project is considered a module when it has a `module:`
+  /// descriptor. A Flutter module project supports integration into an
+  /// existing host app, and has managed platform host code.
   ///
-  /// Such a project can be created using `flutter create -t application`.
-  bool get isApplication => _flutterDescriptor.containsKey('application');
+  /// Such a project can be created using `flutter create -t module`.
+  bool get isModule => _flutterDescriptor.containsKey('module');
 
   /// True if this manifest declares a Flutter plugin project.
   ///
@@ -130,21 +130,21 @@
   bool get isPlugin => _flutterDescriptor.containsKey('plugin');
 
   /// Returns the Android package declared by this manifest in its
-  /// application or plugin descriptor. Returns null, if there is no
+  /// module or plugin descriptor. Returns null, if there is no
   /// such declaration.
   String get androidPackage {
-    if (isApplication)
-      return _flutterDescriptor['application']['androidPackage'];
+    if (isModule)
+      return _flutterDescriptor['module']['androidPackage'];
     if (isPlugin)
       return _flutterDescriptor['plugin']['androidPackage'];
     return null;
   }
 
   /// Returns the iOS bundle identifier declared by this manifest in its
-  /// application descriptor. Returns null, if there is no such declaration.
+  /// module descriptor. Returns null if there is no such declaration.
   String get iosBundleIdentifier {
-    if (isApplication)
-      return _flutterDescriptor['application']['iosBundleIdentifier'];
+    if (isModule)
+      return _flutterDescriptor['module']['iosBundleIdentifier'];
     return null;
   }