Use iptables pipes in BandwidthController startup.

Most of BandwidthController startup is already using
iptables-restore, but some commands (notably listing the costly
chains so they can be flushed by flushCleanTables) still
use iptables.  Move these to use execIptablesRestoreWithOutput.

Test: netd_unit_test passes
Bug: 34873832
Change-Id: Ib0741a99a2605cd6934186fd4e5364331a4eab5a