init: handle sys.powerctl immediately

Currently if a process sets the sys.powerctl property, init adds this
property change into the event queue, just like any other property.
The actual logic to shutdown the device is not executed until init
gets to the action associated with the property change.

This is bad for multiple reasons, but explicitly causes deadlock in
the follow scenario:

A service is started with `exec` or `exec_start`
The same service sets sys.powerctl indicating to the system to
shutdown
The same service then waits infinitely

In this case, init doesn't process any further commands until the exec
service completes, including the command to reboot the device.

This change causes init to immediately handle sys.powerctl and reboot
the device regardless of the state of the event queue, wait for exec,
or wait for property conditions.

Bug: 37209359
Bug: 37415192

Test: Init reboots normally
Test: Update verifier can reboot the system
Change-Id: Iff2295aed970840f47e56c4bacc93001b791fa35