Better timestamp for shutdown and startup events.

Fix the bug that during android P to Q upgrade, DEVICE_SHUTDOWN
event mistakenly gets a newer timestamp than DEVICE_STARTUP event.

Bug: 139656029
Test: flash the device with android P, then upgrade to android Q,
then "adb shell dumpsys usagestats", observe DEVICE_SHUTDOWN
is before DEVICE_STARTUP.

Change-Id: I5429fe7648529be8379270adefd26bd98a31357c