server: Add custom implementation for tethering and statistics

Enable custom network statistics plugin framework. Use platform
specific custom implementation to compute network stats
and enable V6 tethering.
Change-Id: Ife509307e332a8042b6fa920b3a79fb4c656e034

Change-Id: I55d60a62b09197f6792e8723bba9d5d81a4f8a47

TetherController: fix forwarding request aggregation logic
Only enable forwarding on first request.
Only disable forwarding after last request is released.
Change-Id: Iaa489bafe484649656c3f8ee6795cc6ca5008047

Change-Id: Ife509307e332a8042b6fa920b3a79fb4c656e034
Related CR's: 823252 878013 968921
CRs-Fixed: 994082