Avoid using srand/rand

The newlib implementation of `srand` calls `malloc` which is a no-go in some applications.

https://github.com/micro-ROS/rmw_microxrcedds/blob/a698d4e9fc7f6780c44cc900a279d3f2d0c90856/rmw_microxrcedds_c/src/rmw_init.c#L114

Start at rmw_microxrcedds_c/src/rmw_init.c around line 114 and inspect how srand/rand is used during initialization. Identify an approach that avoids the newlib malloc dependency, then verify the initialization path still behaves correctly in the affected applications.

Written by the indexing model from the issue text.