Additional code for server name in DHCP packets.

- Add the hostname to DhcpResultsParcelable.
- Don't store the server hostname if option overload is in use,
  as this is not valid.
- Add unit tests.

Bug: 120584519
Bug: 127423755
Test: atest NetworkStackTests
Test: atest FrameworksNetTests
Change-Id: I47d9d53d1fb58968322cc8b071a44fbc8f7156e1