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>
(cherry picked from commit c1fc90c1724427d8e926172ed258e05ff63b8e26)
Reviewed-on: https://weave-review.googlesource.com/1400