samples: pm: fix init priority after init level change

Moving from APPLICATION level to POST_KERNEL and keeping both parent and
child on the same level is causing problems, so fix this by putting them
on different priorities in POST_KERNEL.

Also, this sample runs fine in qemu, so make it the integration
platform.

Fixes #62772

Signed-off-by: Anas Nashif <anas.nashif@intel.com>