connectivity-service: fix/improve unique hostname

ConnectivityService should set the net.hostname property to either
the current DEVICE_HOSTNAME or android-ANDROID_ID or leave net.hostname
unchanged if already set. However, the logic was flawed such that if
DEVICE_HOSTNAME was empty but net.hostname was not, net.hostname would
always be set to an empty string, leading to DHCP breakage later on.

The logic has been fixed, clarified and improved such that:
net.hostname will only be changed if it is empty. If net.hostname is
empty, it will be set to either (in order): the nonempty value of
DEVICE_HOSTNAME, android-ANDROID_ID or android-r-RANDOM_NUMBER. The last
option is an addition to have a sensible fallback in case both
DEVICE_HOSTNAME and ANDROID_ID are empty.

The random number is generated by java.util.Random, because I consider
cryptographically strong randomness unnecessary here and think possible
blocking in SecureRandom might be undesirable.

Thanks to stargo for pointing me to the right place to edit.
Thanks to Ethan Chen for his stylistic advice.

Change-Id: I603ab4d6a265456bf4fa310939965cfa677fc881