Expose empty PackageDataSnapshot as system API.

`PackageDataSnapshot` is in the parameter list of ART Services APIs
(go/art-services-apis). We need at least an empty class to unblock us.

Bug: 177273468
Test: m update-api
Change-Id: I292f1847ba9b07fc9096b583571cec5e4526b441

services/api/current.txt

@@ -54,6 +54,13 @@ package com.android.server.pm {
 
 }
 
+package com.android.server.pm.snapshot {
+
+  public interface PackageDataSnapshot {
+  }
+
+}
+
 package com.android.server.role {
 
   public interface RoleServicePlatformHelper {

services/core/java/com/android/server/pm/snapshot/PackageDataSnapshot.java

@@ -16,6 +16,7 @@
 
 package com.android.server.pm.snapshot;
 
+import android.annotation.SystemApi;
 import android.content.pm.PackageManagerInternal;
 
 import com.android.server.pm.Computer;
@@ -28,6 +29,9 @@ import com.android.server.pm.PackageManagerService;
  *
  * Usage inside {@link PackageManagerInternal} and related should cast the object instance to
  * a {@link Computer} to access data.
+ *
+ * @hide
  */
+@SystemApi(client = SystemApi.Client.SYSTEM_SERVER)
 public interface PackageDataSnapshot {
 }