init: Reboot after timeout passes during reboot

There is currently a timeout for reboot, however if the system gets
stuck, particularly during file system operations, there is no safety
mechanism that guarantees the system will still reboot.

This change does all of the optional reboot steps in a separate thread
and waits for this thread with a timeout, such that if the reboot
steps get hung, the system is guaranteed to still reboot.

This is specific to 'reboot'.  Shutdown continues to run unbounded to
run fsck.

Bug: 72781711
Test: Reboot devices hitting and not hitting this timeout
Change-Id: Id5e1b3693bab00602177e28b9b662e1499c32961