RouteController: use fixed priority for ip rule

Use fixed priority when adding source policy rules

CR-fixed: 613222
Change-Id: I3182777db6b665c6adc94f65e91b429aac733875
netd: Use append to add routes to secondary routing table

Use <ip route append> instead of <ip route add>
to add routes to the secondary routing table. This
is because adding a route with same v6 address but
different device name fails by using <ip route add>

CRs-Fixed: 618837
Change-Id: Id880e1a155856a5a3afad75135091a17ace07414
Netd: Do not reset routing policies

Stop flushing of all routing policies when resetting to default rule
configuration. This is detrimental to other policy routes set on the
system and creates a conflict. Tethering related rules are already
removed by NatController so this step is unnecessary.

Change-Id: Id9026c089262e37fffe6e5db8eb8e0ccf3338c94