Make ACTION_USER_REMOVED intent a SystemApi.

Fixes: 36645106
Test: cts-tradefed run singleCommand cts-dev -m CtsSignatureTestCases -t android.signature.cts.IntentTest
Change-Id: I4230791df73549f147dc048d152a05e9eed5bc63
diff --git a/core/java/android/content/Intent.java b/core/java/android/content/Intent.java
index 7890a96..e617816 100644
--- a/core/java/android/content/Intent.java
+++ b/core/java/android/content/Intent.java
@@ -3051,6 +3051,7 @@
      * {@link android.Manifest.permission#MANAGE_USERS} to receive this broadcast.
      * @hide
      */
+    @SystemApi
     public static final String ACTION_USER_REMOVED =
             "android.intent.action.USER_REMOVED";