DHCPv4 anonymity profile: support skipping hostname option

Per https://tools.ietf.org/html/rfc7844#section-3.7

   When using the anonymity profile, DHCP clients SHOULD NOT send the
   Host Name option.

Once the net.hostname property is removed and ensured unset via CTS
this option will no longer be sent.

Test: as follows
    - build, flashed
    - connected to v4-only and dualstack networks, verified v4 works
    - android.net.dhcp.DhcpPacketTest passes
    - setprop net.hostname "" and verified no hostname option
Bug: 30979015
Change-Id: Ieab3f04ea38e176d9147bb788742a5397762c2e4