LE: Fix limited advertising not stopping after timeout

When starting to advertise with a given timeout, the alarm did not fire
and thus not stop the advertising. This patch switchs from the new alarm
system to use BTU timers.

Also fixes a bug in the oneshot timer handling where adding a new timer
with a short timeout value would not actually restart the timer to pull
in the deadline.

Bug: 16988160
Change-Id: Ia556562675636be440ddca7682ac7d092bc0b48b