Ensure trust agents are only provided by platform packages

Prevents the TrustManagerService from binding to any non-platform
trust agents and updates the SampleTrustAgent to use the new
permission.

Bug: 15287044
Change-Id: I38c3deebd5ad359075e68f52d19417bab5ab43db
diff --git a/core/res/AndroidManifest.xml b/core/res/AndroidManifest.xml
index 14141d7..e9d9b14 100644
--- a/core/res/AndroidManifest.xml
+++ b/core/res/AndroidManifest.xml
@@ -2610,6 +2610,13 @@
                 android:label="@string/permlab_trust_listener"
                 android:description="@string/permdesc_trust_listener" />
 
+    <!-- Allows an application to provide a trust agent.
+         @hide For security reasons, this is a platform-only permission. -->
+    <permission android:name="android.permission.PROVIDE_TRUST_AGENT"
+                android:protectionLevel="signatureOrSystem"
+                android:label="@string/permlab_provide_trust_agent"
+                android:description="@string/permdesc_provide_trust_agent" />
+
     <!-- Must be required by an {@link
          android.service.trust.TrustAgentService},
          to ensure that only the system can bind to it. -->