Created a firewall chain for power save mode.

When power-save mode was first implemented, there were no firewall rules
on netd, so the solution was to make all network interface metered and
re-use the bw_penalty_box chain.

This change removes that workaround by creating a explicit fw_powersave
chain, whose behavior is similar to fw_dozable (in fact, it reuses some
of its code); such change not only makes network restrictions on
power-save mode simpler, but it also allows to optimze how the restrict
network rules are changed (which will be done in a separate change).

BUG: 27127112
BUG: 26685616
Change-Id: I7f7a7b1c1855e916c6651ad90da29fe187a7bea2