bootstat: fix test regression (exec_background)

Deal with regression from 557a9d40
where bootstat was moved to exec_background and actions delayed
to improve boot time.  Add a 1 second sleep rather than trying to
inspect the dmesg (debug only) for clues to the end of bootstat.

Test: system/core/bootstat/boot_reason_test.sh
Bug: 63736262
Change-Id: I87dfb6a07130112bf51c367632967efa53ea2534