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: I5e9fee5471740f5d7c001e9e98017f32be452344
Merged-In: I60071bc029d25485bf204cfd3a8cebd538ca12b6
(cherry picked from commit 681f33cc)