sepolicy for vendor overlay

Make /(product|system/product)/vendor_overlay/<ver> have the vendor
file context.

If vendor_overlay requires to mount on the vendor directories other
than 'vendor_file', the contexts must be defined in the device
specific sepolicy files.

Bug: 119076200
Test: build and check if the files are overided and have the required
sepolicy contexts.

Change-Id: I69ed38d4ea8e7d89f56865b1ca1e26f290e9892d