pm: adjust busy check API call return types and naming

Busy check APIs now return boolean type. Due to that change, the
function names have also been adjusted. The common name pattern for
boolean check type APIs is "PREFIX_is_CONDITION". For example,
"pm_device_is_busy".  pm_device_busy_check has been renamed to
pm_device_is_busy and pm_device_any_busy_check to pm_device_is_any_busy.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>