alarmtimer: set power off alarm to be triggered on time

As the new design of power-off alarm, it's not necessary to boot up
the phone 2 minutes earlier than the actual power-off alarm time. So
remove this limit here.

Change-Id: I10c47d191d62452a14253db00cb8922368aa9216
Signed-off-by: lijuang <lijuang@codeaurora.org>