init: set class core for rmt_storage

Fix the modem crash on encryption phone
There is an efs sync problem because of class type of rmt_storage service.
When encrypting phone, modem crash may occur due to not being able to sync
properly between AP and CP. So, rmt_storage class should be a core class.

Change-Id: I266994f4d5af39a787dee8c6223596d984db0be5