Fix race condition between lockNow() and updateLockscreenTimeout

If updateLockscreenTimeout gets called before the Runnable queued
from lockNow gets executed, lockNow request will be ignored. Fix
this by not clearing out the runnable if it's pending lock request.

Test: Switch user, ensure lockscreen comes up
Bug: 161149543
Change-Id: Ie486396fd7328edf8ca0912df92524bb82a1fb7f