Disable signal handling in timer thread

When the host application sets up a custom signal handler thread
only that thread is supposed to handle the signals. But heaptrack
spawns up the timer thread on initialization, i.e. when it
first encounters a memory allocation. This usually happens before
the host application has setup the signal mask. As such, our timer
thread would still receive e.g. SIGTERM and then shutdown the whole
application.

Now, we disable signal handling in the helper thread to fix this
behavior for applications that rely on it.

BUG: 378494