Ensure DHCP requests have nonempty hostnames

If the net.hostname property is empty or not found, Android issues
DHCP requests with an empty hostname field (option 12). RFC2132 section
3.14 prescribes a minimum length of 1 byte for the hostname.

To fix this, a generic 'android-dhcp' hostname is used as a fallback.

Change-Id: I7a92e7295f72b3255e4fba6498fb4039906eb702