netd: IPV6 tethering support

- Start NDP proxy daemon when tethering is enabled
- Add new interface commands "add_upstream" and "remove_upstream".
  Used to provide the complete set of interfaces to be proxied to
  the NDP proxy daemon
- Enable ipv6 forwarding
- After repeatedly toggling tethering/Wifi hotspot on and off, hotspot
  may not turn on anymore. It appears untethering is unsuccessful and
  this eventually leads to a timeout and a framework reboot. The NDP
  proxy daemon appears to be started multiple times even though there
  is no ipv6 upstream interface present. There appears to be a signifcant
  number of calls to fork and exec this daemon even though this
  functionality is not needed in this case. Fix this overhead by invoking
  the daemon only if there are atleast 2 interfaces to establish a bridge
  and proxy the NDP messages.

Conflicts:
	server/TetherController.cpp

CRs-fixed: 665103
Change-Id: I89170f8acd60b949ede749f353acd27363ff5798
Add support for ipv6 tethering stats

Requirement: Support for ipv6 tethering stats

Current implementation does not have support
for calculating the Ipv6 tethering  traffic

Solution:
Parse the ipv6 tether counter chains output
Update the output data to NetworkManagementService

Change-Id: I7d626b5697f5aa07c0266032a07be8f70a17f4e3
CRs-Fixed: 690173
netd: add support for ipv6 tethering when using non default apn

When tethering is enabled and a non default apn is used, a policy
route table is created per upstream interface. The table number is
the sum of the interface index and  offset from BASE_TABLE_NUMBER.
There is a rule added per upstream interface which is specific for
the tethered traffic which looks up this table.
This patch adds the capability to pass this table name specific
to the upstream interface to the router advertisement daemon. The
route advertisement daemon will then add the rule which looks up
this table for the ipv6 traffic.

Removed the function addV6RtrAdvIface and removeV6RtrAdvIface
and replaced it with configureV6RtrAdv which has no arguments since
the arguments of the former functions were not being used.

CRs-fixed: 654872
Change-Id: I2845558068ba53032fbbc6be618ac64cfed63c6e