tethering: offload: Netlink Req

Send netlink request over fd for offload config before
completing init sequence. Provides existing conntrack
entries to IPA. Resolves issue where there are NAT
misses in IPA due to IPA only having the conntrack
entries added after tethering starts.

Bug: 149109043
Test: OffloadHardwareInterfaceTest
Original-Change: https://android-review.googlesource.com/1290954
Merged-In: Iaf3e847e92f205b55f10fa85c17b9f3995d52099
Change-Id: Iaf3e847e92f205b55f10fa85c17b9f3995d52099