In cases when a device upgrades to system-as-root from O to P, it needs a mount
point for an already existing partition that is accessed by both system and
vendor.

Devices launching with P must not have /mnt/vendor accessible to system.

Bug: 78598545
Test: m selinx_policy
Change-Id: Ia7bcde44e2b8657a7ad9e0d9bae7a7259f40936f