Specify correct install location for vendor_ramdisk prebuilt kernel module

vendor_ramdisk prebuilt kernel files are installed under
`vendor_ramdisk/lib/modules` regardless of the product configuration in
make. This change matches the behavior and allows the kernel files to be
installed in the correct location.

Test: m soong_generated_vendor_ramdisk_filesystem_test
Bug: 381104942
Change-Id: Id2086de95f0554c849c1667ce190274232db6b99