system_dlkm: add unguarded mount point

Always create system_dlkm mountpoint like
vendor_dlkm and odm_dlkm; as some CF mixed
tests are using the dynamic partition to boot
with builds with no system_dlkm which causes
avd boot failures.

Bug: 217511547
Test: TH
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>
Change-Id: Ibf32127df22898706db02400addaa0f1193c105c