TaskOrganizer: Add WINDOWING_MODE_MULTI_WINDOW support

Also add small test activity.

Bug: 139371701
Test: TaskOrganizerTests. TaskOrganizerMultiWindowTest.
Change-Id: I0bea25a5c8eef25fba88a0d056ef464035749a59
diff --git a/tests/TaskOrganizerTest/AndroidManifest.xml b/tests/TaskOrganizerTest/AndroidManifest.xml
index 0cb6c10..a77d7ee 100644
--- a/tests/TaskOrganizerTest/AndroidManifest.xml
+++ b/tests/TaskOrganizerTest/AndroidManifest.xml
@@ -19,5 +19,11 @@
       <service android:name=".TaskOrganizerPipTest"
                android:exported="true">
       </service>
+      <activity android:name="TaskOrganizerMultiWindowTest" android:label="TaskOrganizer MW Test">
+        <intent-filter>
+          <action android:name="android.intent.action.MAIN"/>
+          <category android:name="android.intent.category.LAUNCHER"/>
+        </intent-filter>
+      </activity>
     </application>
 </manifest>