OWNERS: Trim ACK team members

Since OWNERS file is mostly present to reject changes to the core kernel
and the only time we give OWNER +1s in this tree is for merges from
AOSP, lets remove most ACK team members and add a few people working
on Pixel 2021 GKI.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Change-Id: I54fca16bdc158c2b8a1cda16703add42b8b22ec3
(cherry picked from commit db86e497)

OWNERS_core

-adelva@google.com
-gregkh@google.com
-maennich@google.com
 saravanak@google.com
-tkjos@google.com
 willmcvicker@google.com
-surenb@google.com
+elavila@google.com
\ No newline at end of file