Condition Refactor.

This changelist modifies conditions in two ways:
1. Conditions now can be unset. In the unset state, the condition is
no longer considered when evaluating whether all conditions are met
by the monitor.
2. The overriding feature of a condition is a construction time
decision.

Test: atest ConditionMonitorTest
Test: atest ConditionTest
Bug: 252888175
Change-Id: If254657ef87d12778850a44e1e99fe69d0460476