Require version_code parameter to be an Integer for Android hotfixes

When I created the hotfix branch for Simplenote Android 2.10.1, I
erroneously replied "2.10.1" to the UI prompt for a version code value.

I didn't notice my mistake and pushed the branch, @theck13 had to fix it
in his PR, see
Automattic/simplenote-android@cf3c979

Author: mokagio

lib/fastlane/plugin/wpmreleasetoolkit/actions/android/android_bump_version_hotfix.rb

@@ -41,7 +41,8 @@ def self.available_options
                                        is_string: true),
           FastlaneCore::ConfigItem.new(key: :version_code,
                                        env_name: "FL_ANDROID_BUMP_VERSION_HOTFIX_CODE",
-                                       description: "The version of the hotfix"),
+                                       description: "The version of the hotfix",
+                                       type: Integer),
           FastlaneCore::ConfigItem.new(key: :previous_version_name,
                                        env_name: "FL_ANDROID_BUMP_VERSION_HOTFIX_PREVIOUS_VERSION",
                                        description: "The version to branch from",