Rename IDhcpLeaseCallbacks to IDhcpEventCallbacks for more generic.

This change renames IDhcpLeaseCallbacks, easier to expand in the
future and adds a new method onNewPrefixRequest which will be called
on DHCP server receives DHCPDECLINE message and request a new IPv4
prefix from IpServer.

Bug: 130741856
Test: atest NetworkStackTests NetworkStackNextTests
Change-Id: Ia7b08bb1e7feda08ffe9fbb8885dbc8b7bdd66c9