add a target for android-x64 (#3224)

* add a target for android-x64

* update armeabi-v7a to x86_64
diff --git a/packages/flutter_tools/lib/src/build_configuration.dart b/packages/flutter_tools/lib/src/build_configuration.dart
index 170c4bc..cce4248 100644
--- a/packages/flutter_tools/lib/src/build_configuration.dart
+++ b/packages/flutter_tools/lib/src/build_configuration.dart
@@ -21,6 +21,7 @@
 
 enum TargetPlatform {
   android_arm,
+  android_x64,
   ios,
   darwin_x64,
   linux_x64