Allow product partition to have vendor_overlay

Most vendor_overlay files are product specific that are not allowed
to be installed on system partition.
To install those files on product partition, product/vendor_overlay
is added to the vendor_overlay source directory list.
If the same files are provided from both partitions, files on product
partition will be used.

Bug: 119850553
Test: Build and boot to check if the vendor_overlay works
Change-Id: I1ae97cb5c9dd66d1da5c9eaa3133426d2ba77471