Add OWNERS in system/sepolicy

Owners are selected from top CL approvals or owners.
They will be suggested to review/approve future CLs.

Test: build/make/tools/checkowners.py -c -v OWNERS
Change-Id: I3d7f4c06209c22dea0d824429d68997f7179985f

OWNERS

+nnk@google.com
+jeffv@google.com
+klyubin@google.com
+dcashman@google.com
+jbires@google.com
+sspatil@google.com