init: reboot: Kick off RebootMonitorThread() upon sys.shutdown.requested

The ShutdownThread in Android performs a shutdown/reboot roughly like
this:
1. In beginShutdownSequence(), create a dialog from which the user
   can't escape telling the user that the system is shutting down (or
   rebooting).
2. Fork off a thread, which doesn't seem to have any sort of watchdog
   watching to see if it hangs.
3. In the thread, notify a bunch of things about the shutdown. Some of
   what it does is supposed to timeout after ~10 seconds but it's
   unclear if every action that might be slow or might hang has a
   timeout.
4. Call into the `init` process to actually perform the shutdown.

While activities in the thread that are expected to be long-running do
have timeouts, there is no overall watchdog making sure that something
in the thread doesn't unexpectedly hang and leave the user with the
shutdown dialog up but no way out. There _is_ a watchdog thread
created once we get into the `init` process but since init doesn't
know about the reboot until step #4 there is a bunch of stuff that's
not covered.

Make the watchdog cover everything by kicking `init`'s watchdog off
earlier. This can be done by noting that early in the ShutdownThread
it sets a property "sys.shutdown.requested". The `init` process can
note a set of this property and kick the watchdog off earlier.

Bug: 409835922
Test: With ag/32996886, various shutdown types trigger proper watchdog
Change-Id: Ie161224360e108040805ebe7a33ced64b408befd