Extract file_contexts from zips if it exists

Update binary will now extract file_contexts if it exists in the
root of the zip to /tmp

Recovery will replace the existing /file_contexts in the ramdisk
if file_contexts exists in the root of the zip.

This ensure that the proper contexts are used during zip installs.

Change-Id: If22c41101868643b67e6dba6177677c078fcd877