Fix miss tracking downstream problem in PrivateAddressCoordinator

The downstream do not be tracked if PrivateAddressCoordinator just
return cached address. Then, PrivateAddressCoordinator would not notify
that downstream if conflict happen.

Also remove the null check in getDownstreamPrefix because:
- An IpServer is only added to mDownstreams by requestDownstreamAddress.
- That method will only add the IpServer to mDownstreams if it has an
IPv4 address.
- As soon as that method returns, the IpServer sets mIpv4Address to the
address that was returned.
- When an IpServer is torn down, mIpv4Address is set to null after
releaseDownstream is called.

So it should never be possible for this to return null.

Bug: 168169687
Test: atest CtsTetheringTest
      atest TetheringTests
Change-Id: Ide5206b013acdc499344e1c839a830c5b245af41