Fix path conflict for vendor_ramdisk_available

... and ramdisk_available modules. If a module is both
vendor_ramdisk_available and ramdisk_available, on a device
that mark recovery_as_boot and move_recovery_resources_to_vendor_boot
simultaneously (and incorrectly),
both will be installed to recovery/root/first_stage_ramdisk. Fix the
path conflict of the two variants by moving the vendor_ramdisk variant
to vendor-ramdisk/first_stage_ramdisk instead.

Also update comments for Vendor_ramdisk_available.

Test: m nothing -j
Bug: 156098440

Change-Id: I2b776b6fd8f5a2c361c0f6a89231e3cebc2646f0