move selinux policy loading APIs to platform libselinux

Bug: 37919668
Bug: 37343404
Test: build and boot

(cherry-pick from commit 43d548e365ec19da42c39dd132691af5cae65086)

Change-Id: I43779180fdd00f430de0557860c6264543b018d6
Merged-In: If0de84db4052c86350001de11fc804d9daca1630
2 files changed