lowmemorykiller: avoid false adaptive LMK triggers

In vmpressure notifier of LMK, shift_adj would have been set
by a previous invocation of notifier, which is not followed by
a lowmem_shrink yet. If this is follwed by a lower vmpressure,
and then by a lowmem_shrink, ALMK still triggers because of the
previous higher vmpressure notification. This is wrong.
Since vmpressure has improved, reset shift_adj to avoid false
adaptive LMK trigger.

CRs-fixed: 893699
Change-Id: I2d77103d7c8f4d8a66e4652cba78e619a7bcef9a
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>