Allow tethering pick prefix from all of private address range

Currently tethering only pick prefix from 192.168.0.0/16. There is no
aviable tethering address if the upstream address is 192.168.x.y/16.
This change allow tethering to pick prefix from any private address
rnages. Now it still pick from 192.168.0.0/16 only to avoid behavior
change. Will have follow up commit to change the configuration.

Bug: 166057846
Bug: 170265597
Test: atest TetheringTests
      atest CtsTetheringTest

Change-Id: Ib6304eb8b4788e9196d0af48e72f00a6bda73a5f