Small tweaks to stop_tracing script

Wait for a second before attempting to signal each traced thread.
Go throught the list of threads one by one and sleep for ~100ms
between each. Also go through the pids in reverse numeric order:
this should ensure that we flush as many log files as possible
before eventually kicking zygote and triggering a soft reboot.

Signed-off-by: Jeremy C. Andrus <jeremya@cs.columbia.edu>