lshal: do not pthread_kill

All of the commands are executed by starting a thread,
and if there is a timeout, sending a SIGINT to the thread,
which invokes pthread_exit from the signal handler. If
pthread_exit is called while the thread is in jemalloc code,
that might cause problems.

To avoid this, we stop calling pthread_kill on the background
helper threads. If they time out, simply ignore the thread and
move on. Although this causes memory leak, the lshal tool is
a debugging tool that is intended to run for a short period of
time, not as a daemon. So this is okay.

Test: lshal_test
Bug: 311143089
Change-Id: I031e5fb6cfc0f10952d10e41d6d1f716ff51dcb3