Add DhcpLeaseCallbacks

The callbacks will be used by Tethering to provide callbacks when DHCP
leases are updated.

The current design only supports one client as Tethering may want to
send callbacks to multiple callers, but DhcpServer is only owned by
Tethering.

Bug: 135411507
Test: atest NetworkStackTests
Change-Id: I1e44221d6fbd1b1f2d0d0057a29c7445af1cdbcf