How can I use reflection for this

I posted a question about a new behavior of Android 5.1 that disables the Backup Service when setting up a device-owner on a device here...

A possible solution could be (I guess) to use reflection to fix the issue: I could find some reflection examples using a particular method in a hidden class, but this case looks more complicated, with a constructor that uses another hidden class (ServiceManager) etc. and I dunno how to do this...

The code that is annoying me is located in DevicePolicyManagerService.java (can be found here) on lines 3739 to 3749:

long ident = Binder.clearCallingIdentity();
try {
    IBackupManager ibm = IBackupManager.Stub.asInterface(
        ServiceManager.getService(Context.BACKUP_SERVICE));
    ibm.setBackupServiceActive(UserHandle.USER_OWNER, false);
} catch (RemoteException e) {
    throw new IllegalStateException("Failed deactivating backup service.", e);
} finally {
    Binder.restoreCallingIdentity(ident);
}

And my goal would be to re-enable the Backup Service, ideally this would call something like:

ibm.setBackupServiceActive(UserHandle.USER_OWNER, false);

Could you help me to do this ?