Define rcode=12~15 (UNASSIGNED) in rcode enum type

RCODE   Name
11      NS_R_MAX
12      NS_R_UNASSIGNED12
13      NS_R_UNASSIGNED13
14      NS_R_UNASSIGNED14
15      NS_R_UNASSIGNED15
16      NS_R_BADVERS

According the stats data of Dns Resolver module,
Some DNS Servers might return undefined code to devices.
Without the enum definition, that would be noise for our dashboard

Bug: 150194702
Test: Manual test with statsd_testdrive
Change-Id: I9c9897105092e05787b972e731ded648e9101628