Also mount /sys/fs/selinux

This mount point is required for apexd, which uses libselinux to adjust
label of the compressed APEXes that are decompressed as part of the
postinstall hook.

Bug: 284277137
Test: m dist
Test: system/update_engine/scripts/update_device.py out/dist/ota.zip
Change-Id: Ia1a65a4e69b27351941a290210004a93470ae87f
Merged-In: Ia1a65a4e69b27351941a290210004a93470ae87f
(cherry picked from commit 56fd71b0)