add a --deploy flag to build apk (#3249)

* add a --deploy flag to build apk

* update command description

* use an enum instead of a bool param for build variants

* rename buildForDeploy flag to buildVariant

* review comments

packages/flutter_tools/lib/src/build_configuration.dart

diff --git a/packages/flutter_tools/lib/src/build_configuration.dart b/packages/flutter_tools/lib/src/build_configuration.dart
index cce4248..38bb16408 100644
--- a/packages/flutter_tools/lib/src/build_configuration.dart
+++ b/packages/flutter_tools/lib/src/build_configuration.dart
@@ -14,6 +14,20 @@
   debug,
 }
 
+/// The type of build - `develop` or `deploy`.
+///
+/// TODO(devoncarew): Add a `profile` variant.
+enum BuildVariant {
+  develop,
+  deploy
+}
+
+String getVariantName(BuildVariant variant) {
+  String name = '$variant';
+  int index = name.indexOf('.');
+  return index == -1 ? name : name.substring(index + 1);
+}
+
 enum HostPlatform {
   mac,
   linux,