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
Merged-In: I9c9897105092e05787b972e731ded648e9101628
Change-Id: I26bb7adfea7e9bd2ba73fdad4cc630bf4e3f8f80
(cherry picked from commit 761c5755)