libweave: Change backup polling interval back to 30 minutes The interval has been changed to 30 seconds by mistake when performing code refactoring. It should be 30 minutes. BUG: 25176346 Change-Id: I0ceb1625f733ef10f74b3c2844d4c5b30515a666 Reviewed-on: https://weave-review.googlesource.com/1390 Reviewed-by: Vitaly Buka <vitalybuka@google.com>