Make purge+throttled feature work with memory coordinator
Purge+throttled (previously called purge+suspend) used to depend on
MemoryCoordinatorClient::OnStateChange() and that's why we couldn't enable
purge+throttled and memory coordinator at the same time. As of
crrev.com/2695923003 all purging logic has moved to
MemoryCoordinatorClient::OnPurgeMemory(), which can be used by both
memory coordinator and purge+throttled feature. Both features don't have to be
mutually exclusive anymore.