oomd: Fix Killed signal reason being lost (#40689)

Emitting "oom" doesn't mesh with the org.freedesktop.oom1.Manager
Killed() API contract, which defines "memory-used" and "memory-pressure"
as possible reasons. Consumers that key off reason thus will either lose
policy attribution or may reject the unknown value completely.

Plumb the reason through so it is visible to consumers.