Don't completely stop offload if setting data limit fails.

Currently, if setting a data limit fails, we completely stop
offload in order to avoid data overages. However, the next thing
we do is try to fetch the stats and crash, because once offload
is stopped all our local state is cleared.

Fix this by fetching stats before we stop offload.

Bug: 29337859
Bug: 32163131
Bug: 64867836
Test: OffloadControllerTest passes
Test: no crash when disabling wifi tethering with BT tethering active
Change-Id: I260f5450f8b67f055983af68fb23a5f3cfc0bc69