Support BOARD_SYSTEM_DLKM_SRC build config

In aosp/2323114 , BOARD_SYSTEM_DLKM_SRC is removed in favor of
BOARD_SYSTEM_KERNEL_MODULES . However, BOARD_SYSTEM_DLKM_SRC preserves
directory structure of input kernel modules, where as
BOARD_SYSTEM_KERNEL_MODULES flattens everything by copying all modules
into the same directory. This causes some issues as vendor_dlkm that
depend on system_dlkm modules now need to update their modules.dep file
due to changes in path. For backward compatibility, add support
for BOARD_SYSTEM_DLKM_SRC .

When both BOARD_SYSTEM_DLKM_SRC and BOARD_SYSTEM_KERNEL_MODULES are
set, BOARD_SYSTEM_KERNEL_MODULES will be preferred. The priority is arbitrary,
but we need a defined ordering to prevent double copying.

Test: th
Bug: 267014648
Change-Id: Ib2f5229af02ea2b7c0d96da70afed9118f484c97