Roll forward to cl/314747126

Change-Id: I858d3227b44ca6d727b39fae95358df58fce9c1b

cpp/core_v2/core.cc

@@ -12,6 +12,8 @@ namespace location {
 namespace nearby {
 namespace connections {
 
+constexpr absl::Duration Core::kWaitForDisconnect;
+
 Core::~Core() {
   CountDownLatch latch(1);
   router_.ClientDisconnecting(