Add QuirkEvent for PD provisioned

Since other attempts at creating a provisioning mode metric are not moving forward, this just takes the much simpler approach of adding a quirk event instead. This is trivial to integrate in the current system, and will at least provide some metrics about whether the PD code is actually working.

Change-Id: I86568316cd979cd1e9e740713c2a98697fa197cb
Test: TH
diff --git a/stats/enums/stats/connectivity/network_stack.proto b/stats/enums/stats/connectivity/network_stack.proto
index a375202..c465f07 100644
--- a/stats/enums/stats/connectivity/network_stack.proto
+++ b/stats/enums/stats/connectivity/network_stack.proto
@@ -194,6 +194,7 @@
     QE_APF_OVER_SIZE_FAILURE = 3;
     QE_APF_GENERATE_FILTER_EXCEPTION = 4;
     QE_DHCP6_HEURISTIC_TRIGGERED = 5;
+    QE_DHCP6_PD_PROVISIONED = 6;
 }
 
 enum IpType {