Update examples:

Remove empty MainActivity.java Android example
fix CMake variable BUILD_EXAMPLES -> BUILD_HOST_EXAMPLES
fix some names in example usage messages:
pingable-node -> callbackapi
client -> nonblockingclient
server -> nonblockingserver
pingable-node -> statistics

examples/c/statistics.c

@@ -14,7 +14,7 @@ int main(int argc, char** argv)
 {
     if (argc != 2) {
         printf("\nUsage:\n");
-        printf("pingable-node <net_id>\n");
+        printf("statistics <net_id>\n");
         exit(0);
     }
     long long int net_id = strtoull(argv[1], NULL, 16);   // At least 64 bits