Move REBOOT_READY broadcast into android.scheduling

Moves REBOOT_READY and EXTRA_IS_READY_TO_REBOOT out of the
Intent class. Instead, these will be defined in the
Device Scheduling module.

Also cleans up some documentation.

Bug: 183938070
Test: Builds
Change-Id: Ifd53e95d5eff2aefe025567473c1a769e596392e