Make TrustAgentService a SystemApi

Bug: 16401527
Change-Id: I6d19281600bacac7c8a29afc07c39ca1d461d23c
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 6a7501d..97caf33 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -2730,9 +2730,10 @@
         android:label="@string/permlab_launch_trust_agent_settings"
         android:description="@string/permdesc_launch_trust_agent_settings" />
 
-    <!-- Must be required by an {@link
-         android.service.trust.TrustAgentService},
-         to ensure that only the system can bind to it. -->
+    <!-- @SystemApi Must be required by an {@link
+        android.service.trust.TrustAgentService},
+        to ensure that only the system can bind to it.
+        @hide -->
     <permission android:name="android.permission.BIND_TRUST_AGENT"
                 android:protectionLevel="signature"
                 android:label="@string/permlab_bind_trust_agent_service"