Fix "No file_contexts" warning

Fixed by Loading the file_contexts specified in libselinux, whereas
previously recovery loaded /file_contexts which no longer exists.

Bug: 62587423
Test: build and flash recovery on Angler. Warning is gone.
Test: Wipe data and cache.
Test: sideload OTA
Change-Id: I11581c878b860ac5f412e6e8e7acde811f37870f
(cherry picked from commit 2330dd87)