Allows the caller to specify configuration by TetheringRequest

This is initial work to allow caller to pass their prefered
configuration to start tethering. Caller may able to specify the
downstream interface ipv4 address with dhcp server disabled for
static IP configuration, or able to exempt entitlement check if
they have permission in follow up CL.

Bug: 141256482
Test: -atest TetheringTest
      -ON/OFF wifi tethering

Change-Id: Ic7c3a33195bbd7e72f9b8e73fa148be476b87bf3