Hide KeyChainSystemService.

It's not a public API. Left as-is, it will make to current.txt if anyone touches this folder.

Bug: 266719856
Bug: 266840130
Test: presubmit

Change-Id: Ia686efd6a263930f63a3e5f3221d6975da923967

services/core/java/com/android/server/security/KeyChainSystemService.java

@@ -45,6 +45,7 @@ import com.android.server.SystemService;
  * <p>
  * TODO (b/35968281): take a more sophisticated look at what bits of KeyChain should be inside the
  *                    system server and which make sense inside a system app.
+ * @hide
  */
 public class KeyChainSystemService extends SystemService {