Child Session API changes in configuring and retrieving internal address

This commit:
- creates separate methods for requesting IPv4 and IPv6 internal
  address
- removes method to request internal subnet
- removes methods to request specific DNS and DHCP server
- uses IpPrefix to represent subnet

Bug: 145453636
Bug: 145454043
Bug: 145451419
Bug: 145454818
Test: atest FrameworksIkeTests(all passed)
Test: make update-api && make
Change-Id: Ic3fdccc12e7f2c361637e004e34b27e09d7900a0