init: Set oom_score_adj to snapuserd process

When a process is started as a native service,
oom_score_adj is set to -1000 so that processes
are unkillable by lmkd.

During boot, snapuserd process is not started as a service;
hence, we need to set the oom_score_adj explicitly else in
the event of low memory situation, lmkd can kill the
process thereby device can never boot.

Bug: 234691483
Test: th and OTA on Pixel
Ignore-AOSP-First: cherry-pick from AOSP
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ic2c85aa470522b4bc847a16b4f5cebfc528ed3cf