init: Add reboot timeout handler

In order to prevent device stuck at reboot, we try to create shutdownt
monitor thread with a timeout (default 30s). It will dump init process
and blocked tasks call trace in last kmsg then trigger kernel panic to
reboot device.

Test: reboot device

bug: 128561401
Change-Id: Ieb400ab9fbd983544b61241a4f4b8aa2f4baa863