Check condition presence before updating callbacks.

It is possible that a condition is removed on an execution loop with a
subsequent update callback for that condition queued. This changelist
checks the presence of the condition's callbacks before proceeding to
update them.

Fixes: 237362971
Test: atest ConditionMonitorTest#testUpdateRemovedCallback
Change-Id: If2e4fcc8ff82f51134d91434ad7f2043f531ae77