NetInfo: try to solve crash with releasing _firstTimeReachability

Summary:
@public
We're seeing a crash on line 191 when we do `CFRelease(self->_firstTimeReachability);`. My thinking is that there's a race condition between the deallocation coming from calling `getCurrentConnectivity` twice in a row and the callback coming back.

Reviewed By: PeteTheHeat

Differential Revision: D12982772

fbshipit-source-id: d3d882a074b67a5e547e7f480f561fcaf8d79ec4

Libraries/Network/RCTNetInfo.m

@@ -69,6 +69,13 @@ static void RCTReachabilityCallback(__unused SCNetworkReachabilityRef target, SC
   }
 }
 
+// We need RCTReachabilityCallback's and module methods to be called on the same thread so that we can have
+// guarantees about when we mess with the reachability callbacks.
+- (dispatch_queue_t)methodQueue
+{
+  return dispatch_get_main_queue();
+}
+
 #pragma mark - Lifecycle
 
 - (instancetype)initWithHost:(NSString *)host