Rename tetheroffloadjni to tetheringutilsjni and move setupRaSocket
jni method from NetUtils into tetheringutilsjni.

Bug: 143195885
Test: -build, flash, boot
      -atest TetheringTests
      -Off/On hotspot with ipv6 upstream

Change-Id: Ib68eb6999bbe2d3ca97e68712f22b105311ef800