BandwidthController: fix bad flushing for bw_costly_* tables.

Some of the bw_costly_<iface> rules would not get correctly flushed and
cleared on netd re-start, which would cause a failure when trying to
setup the bw_penalty_box as bw_costly_<iface> would reference it.
The resulting symptom would be that bandwidth could not be re-enabled.

Bug: 10183445
Change-Id: I79a8a73ae52e18b3bff8a58e47ac1aea2454ae63