init: split security functions out of init.cpp

This change splits out the selinux initialization and supporting
functionality into selinux.cpp and splits the security related
initialization of the rng, etc to security.cpp.  It also provides
additional documentation for SEPolicy loading as this has been
requested by some teams.

It additionally cleans up sehandle and sehandle_prop.  The former is
static within selinux.cpp and new wrapper functions are created around
selabel_lookup*() to better serve the users.  The latter is moved to
property_service.cpp as it is isolated to that file for its usage.

Test: boot bullhead
Merged-In: Idc95d493cebc681fbe686b5160502f36af149f60
Change-Id: Idc95d493cebc681fbe686b5160502f36af149f60