Tethering: ensure downstream prefix do not conflict with upstream

- Add New class PrivateAddressCoordinator to coordinate the private
  address conflict problem.
- Downstream prefix would be random in 192.168.0.0/24 ~
  192.168.255.0/24.
- If new upstream prefix is conflict with existing downstream prefix,
  downstream would be kicked out and it would request a new one.
- The last conflict upstream prefixes would be blacklist. Avoid to
select downstream prefix which is conflict with prefixes in blacklist.

Bug: 130879722
Test: -build, flash, boot
      -atest TetheringTests

Change-Id: Ib45b87bcd9eeb5da03fb7ec90b1af9ca53998cf5