lmkd: avoid locking libsigchain into memory

https://android-review.googlesource.com/94851 added an LD_PRELOAD
line to init.environ.rc.in. This has the effect of loading
libsigchain.so into every process' memory space, regardless of
whether it wants it or not.

For lmkd, it doesn't need libsigchain, so it doesn't make any sense
to load it and keep it locked in memory.

Disable noatsecure for lmkd. This sets AT_SECURE=1, which instructs the
linker to not honor security sensitive environment variables such
as LD_PRELOAD. This prevents libsigchain.so from being loaded into
lmkd's memory.

(cherry picked from commit 8a5b28d2)

Change-Id: I39baaf62058986d35ad43de708aaa3daf93b2df4