atrace: make 'lowmemorykiller' directory optional for memreclaim

The 'lowmemorykiller' directory isn't used for lmk tracing on devices that have userspace lmk.

Bug: 110999121
Test: The memreclaim option is available in Traceur on a device with
userspace lmk.

Change-Id: I5dadb5c6c55cb04b6fe079b5f10a3dc35f1ee3f1