Try to set the hop limit to something suitable for 64share

Introduce hopLimit into RaParams so in future the value can be passed
in from the control plane up in Tethering.

Test: as follows
    - built, flashed, booted
    - runtest frameworks-net passes
Bug: 32163131
Change-Id: I8d2f056dcd55d76e89199db1196754fb690250e8