Merge changes from topic 'selinux-stage-merge' into stage-aosp-master

* changes:
  sefcontext_compile: migrate to soong
  move file_context APIs out of libselinux_vendor
  Do not use PCRE2 in libselinux_vendor
  move selinux policy loading APIs to platform libselinux
  refactor: move common parts of libselinux builds into defaults
  Add libselinux_vendor