libweave: Move backoff_entry from libchromeos into libweave

It's used by libweave only.

BUG=brillo:1257
TEST=`FEATURES=test emerge-gizmo libchromeos libweave`

Change-Id: I07aa30a9b758161bf29c8bce7a057550f565dc94
Reviewed-on: https://chromium-review.googlesource.com/293421
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>