init: reboot: Create a function for starting RebootMonitorThread()

We want to be able to start RebootMonitorThread() from more than one
place. Separate the starting to a function and use a "test and set" to
make sure that we only ever start the thread once.

Bug: 409835922
Test: Hack a timeout in and see shutdown/reboot still work.
Change-Id: I9c4f473fb482a0732a27b64a34908df80180ee6e