Support specific client address configuration

If specific config "clientAddr" is set, the DHCP server will only offer
"clientAddr". This is for peer-to-peer use case. If there are multiple
clients for which "clientAddr" is already in use, dhcp server will not
offer other addresses to clients.

Bug: 141256482
Test: manual
      atest NetworkStackNextTests

Merged-In: I96bc24a9c30bfc48dff38c3c4456085694fd381c
Change-Id: I96bc24a9c30bfc48dff38c3c4456085694fd381c