Prefer property_contexts from /system & /vendor

This changes system_properties' initialize_properties to prefer
loading property_contexts from /system/etc/selinux &
/vendor/etc/selinux, while falling back to the pre-existing behavior
of loading from /.

Test: Device with *_property_contexts in / boots up fine, no denials
      to do with properties, getprop -Z lists correct labels.
Test: Device with *_property_contexts in /system & /vendor, but not
      in /, boots up fine, no denials to do with properties,
      getprop -Z lists correct labels.
Test: Device with *_property_contexts in /system & vendor and with
      empty *_property_contexts in / boots up fine, no denials to do
      with properties, getprop -Z lists correct labels.
Bug: 36002573

Change-Id: I15174acdf89ee8f5a96acf1e38a54d4214df51ef