[flutter_tools] ensure x64 is built if target-platforms is not specified (#62096)

In the add-2-app case, users may not specify the android target platforms field, but this only defaults to arm builds. Add x64 to the default list.

Fixes #62095

dev/devicelab/bin/tasks/gradle_plugin_fat_apk_test.dart

@@ -51,6 +51,8 @@ Future<void> main() async {
           'lib/armeabi-v7a/libapp.so',
           'lib/arm64-v8a/libflutter.so',
           'lib/arm64-v8a/libapp.so',
+          'lib/x86_64/libflutter.so',
+          'lib/x86_64/libapp.so',
         ], apkFiles);
 
         checkCollectionDoesNotContain<String>(debugAssets, apkFiles);

packages/flutter_tools/gradle/flutter.gradle

@@ -77,6 +77,7 @@ class FlutterPlugin implements Plugin<Project> {
     private static final List DEFAULT_PLATFORMS = [
         PLATFORM_ARM32,
         PLATFORM_ARM64,
+        PLATFORM_X86_64,
     ]
 
     // The name prefix for flutter builds.  This is used to identify gradle tasks