sepolicy: fix system_file_type and get_prop for coredomians.

As part of new AOSP restriction all the domains which are working
from system partation should have "system_file_type" attribute
else will lead to compile time failure .

For reading / setting any property  we should be using
following macros .
set_prop( domain, property_label)
get_prop( domain, property_label)

So addressing  these as part of new requirments .

Change-Id: I6ef373404640f285a57484024665a42f615ce863