Freezer: fix delayed freezing operation

AMS will compute and apply OomAdj frequently, updating the value
again and again. That makes freezer fall into a loop, removing and
resetting delayed freezing messages. Fix it by dropping those
redundant messages.

Bug: 188189336
Bug: 181278288
Test: Verified apps are frozen 10 min after they become cached.
Change-Id: I83aac961b00db129e0d84f1f4f485012c8aa90d4