move file_context APIs out of libselinux_vendor

Also make sure label_file.c is compiled in all targets that libselinux
is built for.

Bug: 37343404
Bug: 37919668
Test: Build and boot

(cherry-pick from commit 554b7e4e82ee4b23155ade680c985e2f657c208a)

Change-Id: I19183fa192afb29ea75f40c0abf1e684b5550d2a
Merged-In: I49259c6cd6543329fe399fc92e07f16dc54c6395
4 files changed