Allow overriding conditions for Conditions Monitor.

This changelist introduces overriding conditions to Conditions Monitor, which the state of one condition to take precedence over others that are present. To support this feature, the following changes have been made to the Monitor:
- Conditions can now be added and removed to the Monitor post construction.
- Changes to the callbacks and conditions are now handled on the main executor.
- The value for a condition is consulted on every update. Callbacks no longer include the updated value.

Bug: 218744120
Test: atest ConditionMonitorTest
Change-Id: If895732ae9fad120eccabb78207f4555c6236fde