Allow init to skip mounting /product and /product_services

mainline-core is still in the progress of defining interfaces between
/system, /product, /product_services. Before that was completely done,
just updating /system to GSI usually couldn't boot a device.

Therefore, this CL allows GSI to specify a config file, telling init
not to mount /product and /product_services. And fallback to
/system/product and /system/product_services in the GSI.

Bug: 120208657
Test: boots a device with /system/etc/init/config/skip_mount.cfg and
      checks both /product and /product_services are not mounted.

Change-Id: Ibe2abad30b178a6e552fb21e4b30207fc3ef5523