Add TestApi interfaces for window organizers

Enables testing the API surfaces from CTS.

Bug: 149338177
Test: they pass!
Change-Id: I7e1f2852585a10c20d299bd87e9a87f828d06d6a
diff --git a/core/java/android/window/WindowContainerToken.aidl b/core/java/android/window/WindowContainerToken.aidl
new file mode 100644
index 0000000..f22786b
--- /dev/null
+++ b/core/java/android/window/WindowContainerToken.aidl
@@ -0,0 +1,19 @@
+/*
+ * Copyright (C) 2020 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 android.window;
+
+parcelable WindowContainerToken;