Disable capability endpoints when not using default backend

The capability names exposed by these endpoints are implementation
details of the DefaultPermissionBackend. A different implementation may
choose to hide the GlobalPermission enum names from the user entirely,
so it doesn't make sense to expose them.

Change-Id: I69ea38064ccbf1c04a976e7bcb4629ab5d66aeef

java/com/google/gerrit/server/permissions/DefaultPermissionBackend.java

@@ -84,6 +84,11 @@ public class DefaultPermissionBackend extends PermissionBackend {
     return new WithUserImpl(identifiedUser);
   }
 
+  @Override
+  public boolean usesDefaultCapabilities() {
+    return true;
+  }
+
   class WithUserImpl extends WithUser {
     private final CurrentUser user;
     private Boolean admin;