Allow zygote to access dir/file under /vendor/overlay

http://ag/2070347 doesn't allow zygote to read vendor_overlay_file:file
anymore.
But zygote isn't transitioned into idmap when executing idmap_exec. So
we need to allow zygote to access dir/file under /vendor/overlay to
enable idmap_exec run by zygote to read static RRO.

Test: building succeeded and tested a static RRO on sailfish device.
Bug: 37173452
Change-Id: Iec8a6b31d24c225f7819eeb885305f78da73b8e0