cts: Enforce turning off /dev/mem and /dev/kmem

Android kernels should have CONFIG_DEVMEM and CONFIG_DEVKMEM
not set and ueventd should not have any reason to be creating
these character nodes. selinux will block access if these
nodes exist, we need to check if the nodes got created.

Bug: 19549480
Change-Id: I588cef5f494ff33a45f99721a874ec3c1ef92b3f