Clean up UiPreference

Rename UiType as it is a neutral type, not necessarily tied to a user
preference. Move to client extension API package in preparation for
exposing via REST API to the GWT UI. Add a convenience method for
parsing, albeit without Optional as we don't have that in GWT.

Change-Id: Ib2a7061ce2c34bf6d452d060cc045cf65481b208

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/client/UiType.java

@@ -0,0 +1,32 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.client;
+
+public enum UiType {
+  NONE,
+  GWT,
+  POLYGERRIT;
+
+  public static UiType parse(String str) {
+    if (str != null) {
+      for (UiType type : UiType.values()) {
+        if (type.name().equalsIgnoreCase(str)) {
+          return type;
+        }
+      }
+    }
+    return null;
+  }
+}